From b5f596c3ffb655b6e4fee127fa9336c829198b5b Mon Sep 17 00:00:00 2001
From: Felipe Artur <fcardozo@gitlab.com>
Date: Fri, 7 Jul 2017 15:08:49 +0000
Subject: [PATCH] Native group milestones

---
 .../groups/milestones_controller.rb           |  79 ++++++------
 .../projects/milestones_controller.rb         |  28 +++--
 app/finders/issuable_finder.rb                |  17 +--
 app/finders/milestones_finder.rb              |  59 +++++++--
 app/helpers/milestones_helper.rb              |  14 ++-
 app/models/concerns/internal_id.rb            |   3 +-
 app/models/concerns/issuable.rb               |   1 +
 app/models/concerns/milestoneish.rb           |  16 +++
 app/models/dashboard_milestone.rb             |   4 +
 app/models/global_milestone.rb                |  47 +++++--
 app/models/group.rb                           |   1 +
 app/models/group_milestone.rb                 |   4 +
 app/models/milestone.rb                       |  72 ++++++++++-
 app/services/issuable_base_service.rb         |  15 ++-
 app/services/issues/move_service.rb           |  14 ++-
 app/services/milestones/base_service.rb       |   6 +
 app/services/milestones/close_service.rb      |   2 +-
 app/services/milestones/create_service.rb     |   4 +-
 app/services/milestones/reopen_service.rb     |   2 +-
 app/services/milestones/update_service.rb     |   4 +-
 app/views/groups/milestones/_form.html.haml   |  27 ++++
 .../groups/milestones/_milestone.html.haml    |   3 +-
 app/views/groups/milestones/edit.html.haml    |   7 ++
 app/views/groups/milestones/index.html.haml   |   5 -
 app/views/groups/milestones/new.html.haml     |  38 +-----
 app/views/groups/milestones/show.html.haml    |   2 +-
 .../shared/milestones/_milestone.html.haml    |  35 ++++--
 app/views/shared/milestones/_top.html.haml    |  64 ++++++----
 changelogs/unreleased/issue_30126_be.yml      |   4 +
 config/routes/group.rb                        |   2 +-
 ...170723183807_add_group_id_to_milestones.rb |  18 +++
 ...24184243_add_group_milestone_id_indexes.rb |  19 +++
 db/schema.rb                                  |   7 +-
 doc/user/project/milestones/index.md          |  11 +-
 features/group/milestones.feature             |   4 +-
 features/steps/group/milestones.rb            |   9 +-
 lib/api/entities.rb                           |   9 +-
 .../groups/milestones_controller_spec.rb      | 118 +++++++++++++++---
 .../projects/milestones_controller_spec.rb    |  34 +++++
 spec/factories/milestones.rb                  |  22 +++-
 spec/features/groups/milestone_spec.rb        |  28 +++++
 spec/features/milestone_spec.rb               |  23 +++-
 spec/finders/issues_finder_spec.rb            |  17 +++
 spec/finders/merge_requests_finder_spec.rb    |  19 +++
 spec/finders/milestones_finder_spec.rb        |  90 +++++++++++++
 .../api/schemas/public_api/v3/issues.json     |   3 +-
 .../schemas/public_api/v3/merge_requests.json |   3 +-
 .../api/schemas/public_api/v4/issues.json     |   3 +-
 .../schemas/public_api/v4/merge_requests.json |   3 +-
 spec/lib/gitlab/import_export/all_models.yml  |   1 +
 .../import_export/safe_model_attributes.yml   |   1 +
 spec/models/milestone_spec.rb                 |  44 +++++--
 spec/services/issues/move_service_spec.rb     |  65 ++++++++--
 spec/services/issues/update_service_spec.rb   |   8 +-
 .../merge_requests/update_service_spec.rb     |   8 +-
 spec/support/issuable_shared_examples.rb      |  31 +++++
 56 files changed, 937 insertions(+), 240 deletions(-)
 create mode 100644 app/views/groups/milestones/_form.html.haml
 create mode 100644 app/views/groups/milestones/edit.html.haml
 create mode 100644 changelogs/unreleased/issue_30126_be.yml
 create mode 100644 db/migrate/20170723183807_add_group_id_to_milestones.rb
 create mode 100644 db/migrate/20170724184243_add_group_milestone_id_indexes.rb
 create mode 100644 spec/finders/milestones_finder_spec.rb

diff --git a/app/controllers/groups/milestones_controller.rb b/app/controllers/groups/milestones_controller.rb
index 6b1d418fc9a..5c10d7bc261 100644
--- a/app/controllers/groups/milestones_controller.rb
+++ b/app/controllers/groups/milestones_controller.rb
@@ -2,13 +2,13 @@ class Groups::MilestonesController < Groups::ApplicationController
   include MilestoneActions
 
   before_action :group_projects
-  before_action :milestone, only: [:show, :update, :merge_requests, :participants, :labels]
-  before_action :authorize_admin_milestones!, only: [:new, :create, :update]
+  before_action :milestone, only: [:edit, :show, :update, :merge_requests, :participants, :labels]
+  before_action :authorize_admin_milestones!, only: [:edit, :new, :create, :update]
 
   def index
     respond_to do |format|
       format.html do
-        @milestone_states = GlobalMilestone.states_count(@projects)
+        @milestone_states = GlobalMilestone.states_count(group_projects, group)
         @milestones = Kaminari.paginate_array(milestones).page(params[:page])
       end
       format.json do
@@ -22,50 +22,42 @@ class Groups::MilestonesController < Groups::ApplicationController
   end
 
   def create
-    project_ids = params[:milestone][:project_ids].reject(&:blank?)
-    title = milestone_params[:title]
+    @milestone = Milestones::CreateService.new(group, current_user, milestone_params).execute
 
-    if create_milestones(project_ids)
-      redirect_to milestone_path(title)
+    if @milestone.persisted?
+      redirect_to milestone_path
     else
-      render_new_with_error(project_ids.empty?)
+      render "new"
     end
   end
 
   def show
   end
 
+  def edit
+    render_404 if @milestone.is_legacy_group_milestone?
+  end
+
   def update
-    @milestone.milestones.each do |milestone|
-      Milestones::UpdateService.new(milestone.project, current_user, milestone_params).execute(milestone)
+    # Keep this compatible with legacy group milestones where we have to update
+    # all projects milestones states at once.
+    if @milestone.is_legacy_group_milestone?
+      update_params = milestone_params.select { |key| key == "state_event" }
+      milestones = @milestone.milestones
+    else
+      update_params = milestone_params
+      milestones = [@milestone]
     end
 
-    redirect_back_or_default(default: milestone_path(@milestone.title))
+    milestones.each do |milestone|
+      Milestones::UpdateService.new(milestone.parent, current_user, update_params).execute(milestone)
+    end
+
+    redirect_to milestone_path
   end
 
   private
 
-  def create_milestones(project_ids)
-    return false unless project_ids.present?
-
-    ActiveRecord::Base.transaction do
-      @projects.where(id: project_ids).each do |project|
-        Milestones::CreateService.new(project, current_user, milestone_params).execute
-      end
-    end
-
-    true
-  rescue ActiveRecord::ActiveRecordError => e
-    flash.now[:alert] = "An error occurred while creating the milestone: #{e.message}"
-    false
-  end
-
-  def render_new_with_error(empty_project_ids)
-    @milestone = Milestone.new(milestone_params)
-    @milestone.errors.add(:base, "Please select at least one project.") if empty_project_ids
-    render :new
-  end
-
   def authorize_admin_milestones!
     return render_404 unless can?(current_user, :admin_milestones, group)
   end
@@ -74,16 +66,31 @@ class Groups::MilestonesController < Groups::ApplicationController
     params.require(:milestone).permit(:title, :description, :start_date, :due_date, :state_event)
   end
 
-  def milestone_path(title)
-    group_milestone_path(@group, title.to_slug.to_s, title: title)
+  def milestone_path
+    if @milestone.is_legacy_group_milestone?
+      group_milestone_path(group, @milestone.safe_title, title: @milestone.title)
+    else
+      group_milestone_path(group, @milestone.iid)
+    end
   end
 
   def milestones
-    @milestones = GroupMilestone.build_collection(@group, @projects, params)
+    search_params = params.merge(group_ids: group.id)
+
+    milestones = MilestonesFinder.new(search_params).execute
+    legacy_milestones = GroupMilestone.build_collection(group, group_projects, params)
+
+    milestones + legacy_milestones
   end
 
   def milestone
-    @milestone = GroupMilestone.build(@group, @projects, params[:title])
+    @milestone =
+      if params[:title]
+        GroupMilestone.build(group, group_projects, params[:title])
+      else
+        group.milestones.find_by_iid(params[:id])
+      end
+
     render_404 unless @milestone
   end
 end
diff --git a/app/controllers/projects/milestones_controller.rb b/app/controllers/projects/milestones_controller.rb
index c4723c72136..c94384d2a1a 100644
--- a/app/controllers/projects/milestones_controller.rb
+++ b/app/controllers/projects/milestones_controller.rb
@@ -13,20 +13,16 @@ class Projects::MilestonesController < Projects::ApplicationController
   respond_to :html
 
   def index
-    @milestones =
-      case params[:state]
-      when 'all' then @project.milestones
-      when 'closed' then @project.milestones.closed
-      else @project.milestones.active
-      end
-
     @sort = params[:sort] || 'due_date_asc'
-    @milestones = @milestones.sort(@sort)
+    @milestones = milestones.sort(@sort)
 
     respond_to do |format|
       format.html do
         @project_namespace = @project.namespace.becomes(Namespace)
-        @milestones = @milestones.includes(:project)
+        # We need to show group milestones in the JSON response
+        # so that people can filter by and assign group milestones,
+        # but we don't need to show them on the project milestones page itself.
+        @milestones = @milestones.for_projects
         @milestones = @milestones.page(params[:page])
       end
       format.json do
@@ -51,7 +47,7 @@ class Projects::MilestonesController < Projects::ApplicationController
   def create
     @milestone = Milestones::CreateService.new(project, current_user, milestone_params).execute
 
-    if @milestone.save
+    if @milestone.valid?
       redirect_to project_milestone_path(@project, @milestone)
     else
       render "new"
@@ -86,6 +82,18 @@ class Projects::MilestonesController < Projects::ApplicationController
 
   protected
 
+  def milestones
+    @milestones ||= begin
+      if @project.group && can?(current_user, :read_group, @project.group)
+        group = @project.group
+      end
+
+      search_params = params.merge(project_ids: @project.id, group_ids: group&.id)
+
+      MilestonesFinder.new(search_params).execute
+    end
+  end
+
   def milestone
     @milestone ||= @project.milestones.find_by!(iid: params[:id])
   end
diff --git a/app/finders/issuable_finder.rb b/app/finders/issuable_finder.rb
index 7bc2117f61e..d81e9ed17d4 100644
--- a/app/finders/issuable_finder.rb
+++ b/app/finders/issuable_finder.rb
@@ -147,9 +147,17 @@ class IssuableFinder
 
     @milestones =
       if milestones?
-        scope = Milestone.where(project_id: projects)
+        if project?
+          group_id = project.group&.id
+          project_id = project.id
+        end
 
-        scope.where(title: params[:milestone_title])
+        group_id = group.id if group
+
+        search_params =
+          { title: params[:milestone_title], project_ids: project_id, group_ids: group_id }
+
+        MilestonesFinder.new(search_params).execute
       else
         Milestone.none
       end
@@ -331,11 +339,6 @@ class IssuableFinder
         items = items.left_joins_milestones.where('milestones.start_date <= NOW()')
       else
         items = items.with_milestone(params[:milestone_title])
-        items_projects = projects(items)
-
-        if items_projects
-          items = items.where(milestones: { project_id: items_projects })
-        end
       end
     end
 
diff --git a/app/finders/milestones_finder.rb b/app/finders/milestones_finder.rb
index 630c73c2a94..23c42a5f662 100644
--- a/app/finders/milestones_finder.rb
+++ b/app/finders/milestones_finder.rb
@@ -1,12 +1,55 @@
-class MilestonesFinder
-  def execute(projects, params)
-    milestones = Milestone.of_projects(projects)
-    milestones = milestones.reorder("due_date ASC")
+# Search for milestones
+#
+# params - Hash
+#   project_ids: Array of project ids or single project id.
+#   group_ids: Array of group ids or single group id.
+#   order - Orders by field default due date asc.
+#   title - filter by title.
+#   state - filters by state.
 
-    case params[:state]
-    when 'closed' then milestones.closed
-    when 'all' then milestones
-    else milestones.active
+class MilestonesFinder
+  attr_reader :params, :project_ids, :group_ids
+
+  def initialize(params = {})
+    @project_ids = Array(params[:project_ids])
+    @group_ids = Array(params[:group_ids])
+    @params = params
+  end
+
+  def execute
+    return Milestone.none if project_ids.empty? && group_ids.empty?
+
+    items = Milestone.all
+    items = by_groups_and_projects(items)
+    items = by_title(items)
+    items = by_state(items)
+
+    order(items)
+  end
+
+  private
+
+  def by_groups_and_projects(items)
+    items.for_projects_and_groups(project_ids, group_ids)
+  end
+
+  def by_title(items)
+    if params[:title]
+      items.where(title: params[:title])
+    else
+      items
+    end
+  end
+
+  def by_state(items)
+    Milestone.filter_by_state(items, params[:state])
+  end
+
+  def order(items)
+    if params.has_key?(:order)
+      items.reorder(params[:order])
+    else
+      items.reorder('due_date ASC')
     end
   end
 end
diff --git a/app/helpers/milestones_helper.rb b/app/helpers/milestones_helper.rb
index 8c7851dcfc2..f8860bfee99 100644
--- a/app/helpers/milestones_helper.rb
+++ b/app/helpers/milestones_helper.rb
@@ -54,8 +54,10 @@ module MilestonesHelper
   def milestone_class_for_state(param, check, match_blank_param = false)
     if match_blank_param
       'active' if param.blank? || param == check
+    elsif param == check
+      'active'
     else
-      'active' if param == check
+      check
     end
   end
 
@@ -147,4 +149,14 @@ module MilestonesHelper
       labels_dashboard_milestone_path(milestone, title: milestone.title, format: :json)
     end
   end
+
+  def group_milestone_route(milestone, params = {})
+    params = nil if params.empty?
+
+    if milestone.is_legacy_group_milestone?
+      group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: params)
+    else
+      group_milestone_path(@group, milestone.iid, milestone: params)
+    end
+  end
 end
diff --git a/app/models/concerns/internal_id.rb b/app/models/concerns/internal_id.rb
index 5382dde6765..67a0adfcd56 100644
--- a/app/models/concerns/internal_id.rb
+++ b/app/models/concerns/internal_id.rb
@@ -8,7 +8,8 @@ module InternalId
 
   def set_iid
     if iid.blank?
-      records = project.send(self.class.name.tableize)
+      parent = project || group
+      records = parent.send(self.class.name.tableize)
       records = records.with_deleted if self.paranoid?
       max_iid = records.maximum(:iid)
 
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 23cb85600da..13fe9d09c69 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -30,6 +30,7 @@ module Issuable
     belongs_to :updated_by, class_name: "User"
     belongs_to :last_edited_by, class_name: 'User'
     belongs_to :milestone
+
     has_many :notes, as: :noteable, inverse_of: :noteable, dependent: :destroy do # rubocop:disable Cop/ActiveRecordDependent
       def authors_loaded?
         # We check first if we're loaded to not load unnecessarily.
diff --git a/app/models/concerns/milestoneish.rb b/app/models/concerns/milestoneish.rb
index 01599ce49c6..f0998465822 100644
--- a/app/models/concerns/milestoneish.rb
+++ b/app/models/concerns/milestoneish.rb
@@ -70,6 +70,22 @@ module Milestoneish
     due_date && due_date.past?
   end
 
+  def is_group_milestone?
+    false
+  end
+
+  def is_project_milestone?
+    false
+  end
+
+  def is_legacy_group_milestone?
+    false
+  end
+
+  def is_dashboard_milestone?
+    false
+  end
+
   private
 
   def count_issues_by_state(user)
diff --git a/app/models/dashboard_milestone.rb b/app/models/dashboard_milestone.rb
index 646c1e5ce1a..fac7c5e5c85 100644
--- a/app/models/dashboard_milestone.rb
+++ b/app/models/dashboard_milestone.rb
@@ -2,4 +2,8 @@ class DashboardMilestone < GlobalMilestone
   def issues_finder_params
     { authorized_only: true }
   end
+
+  def is_dashboard_milestone?
+    true
+  end
 end
diff --git a/app/models/global_milestone.rb b/app/models/global_milestone.rb
index 538615130a7..c0864769314 100644
--- a/app/models/global_milestone.rb
+++ b/app/models/global_milestone.rb
@@ -2,6 +2,7 @@ class GlobalMilestone
   include Milestoneish
 
   EPOCH = DateTime.parse('1970-01-01')
+  STATE_COUNT_HASH = { opened: 0, closed: 0, all: 0 }.freeze
 
   attr_accessor :title, :milestones
   alias_attribute :name, :title
@@ -11,7 +12,10 @@ class GlobalMilestone
   end
 
   def self.build_collection(projects, params)
-    child_milestones = MilestonesFinder.new.execute(projects, params)
+    params =
+      { project_ids: projects.map(&:id), state: params[:state] }
+
+    child_milestones = MilestonesFinder.new(params).execute
 
     milestones = child_milestones.select(:id, :title).group_by(&:title).map do |title, grouped|
       milestones_relation = Milestone.where(id: grouped.map(&:id))
@@ -28,13 +32,42 @@ class GlobalMilestone
     new(title, child_milestones)
   end
 
-  def self.states_count(projects)
-    relation = MilestonesFinder.new.execute(projects, state: 'all')
-    milestones_by_state_and_title = relation.reorder(nil).group(:state, :title).count
+  def self.states_count(projects, group = nil)
+    legacy_group_milestones_count = legacy_group_milestone_states_count(projects)
+    group_milestones_count = group_milestones_states_count(group)
 
-    opened = count_by_state(milestones_by_state_and_title, 'active')
-    closed = count_by_state(milestones_by_state_and_title, 'closed')
-    all = milestones_by_state_and_title.map { |(_, title), _| title }.uniq.count
+    legacy_group_milestones_count.merge(group_milestones_count) do |k, legacy_group_milestones_count, group_milestones_count|
+      legacy_group_milestones_count + group_milestones_count
+    end
+  end
+
+  def self.group_milestones_states_count(group)
+    return STATE_COUNT_HASH unless group
+
+    params = { group_ids: [group.id], state: 'all', order: nil }
+
+    relation = MilestonesFinder.new(params).execute
+    grouped_by_state = relation.group(:state).count
+
+    {
+      opened: grouped_by_state['active'] || 0,
+      closed: grouped_by_state['closed'] || 0,
+      all: grouped_by_state.values.sum
+    }
+  end
+
+  # Counts the legacy group milestones which must be grouped by title
+  def self.legacy_group_milestone_states_count(projects)
+    return STATE_COUNT_HASH unless projects
+
+    params = { project_ids: projects.map(&:id), state: 'all', order: nil }
+
+    relation = MilestonesFinder.new(params).execute
+    project_milestones_by_state_and_title = relation.group(:state, :title).count
+
+    opened = count_by_state(project_milestones_by_state_and_title, 'active')
+    closed = count_by_state(project_milestones_by_state_and_title, 'closed')
+    all = project_milestones_by_state_and_title.map { |(_, title), _| title }.uniq.count
 
     {
       opened: opened,
diff --git a/app/models/group.rb b/app/models/group.rb
index f29e642ac91..70a4ceeffd8 100644
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -18,6 +18,7 @@ class Group < Namespace
 
   has_many :requesters, -> { where.not(requested_at: nil) }, dependent: :destroy, as: :source, class_name: 'GroupMember' # rubocop:disable Cop/ActiveRecordDependent
 
+  has_many :milestones
   has_many :project_group_links, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :shared_projects, through: :project_group_links, source: :project
   has_many :notification_settings, dependent: :destroy, as: :source # rubocop:disable Cop/ActiveRecordDependent
diff --git a/app/models/group_milestone.rb b/app/models/group_milestone.rb
index 86d38e5468b..65249bd7bfc 100644
--- a/app/models/group_milestone.rb
+++ b/app/models/group_milestone.rb
@@ -16,4 +16,8 @@ class GroupMilestone < GlobalMilestone
   def issues_finder_params
     { group_id: group.id }
   end
+
+  def is_legacy_group_milestone?
+    true
+  end
 end
diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index c0ccbf8e27e..48d00764965 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -18,17 +18,32 @@ class Milestone < ActiveRecord::Base
   cache_markdown_field :description
 
   belongs_to :project
+  belongs_to :group
+
   has_many :issues
   has_many :labels, -> { distinct.reorder('labels.title') },  through: :issues
   has_many :merge_requests
   has_many :events, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
+  scope :of_projects, ->(ids) { where(project_id: ids) }
+  scope :of_groups, ->(ids) { where(group_id: ids) }
   scope :active, -> { with_state(:active) }
   scope :closed, -> { with_state(:closed) }
-  scope :of_projects, ->(ids) { where(project_id: ids) }
+  scope :for_projects, -> { where(group: nil).includes(:project) }
 
-  validates :title, presence: true, uniqueness: { scope: :project_id }
-  validates :project, presence: true
+  scope :for_projects_and_groups, -> (project_ids, group_ids) do
+    conditions = []
+    conditions << arel_table[:project_id].in(project_ids) if project_ids.compact.any?
+    conditions << arel_table[:group_id].in(group_ids) if group_ids.compact.any?
+
+    where(conditions.reduce(:or))
+  end
+
+  validates :group, presence: true, unless: :project
+  validates :project, presence: true, unless: :group
+
+  validate :uniqueness_of_title, if: :title_changed?
+  validate :milestone_type_check
   validate :start_date_should_be_less_than_due_date, if: proc { |m| m.start_date.present? && m.due_date.present? }
 
   strip_attributes :title
@@ -63,6 +78,14 @@ class Milestone < ActiveRecord::Base
 
       where(t[:title].matches(pattern).or(t[:description].matches(pattern)))
     end
+
+    def filter_by_state(milestones, state)
+      case state
+      when 'closed' then milestones.closed
+      when 'all' then milestones
+      else milestones.active
+      end
+    end
   end
 
   def self.reference_prefix
@@ -138,6 +161,8 @@ class Milestone < ActiveRecord::Base
   #   Milestone.first.to_reference(same_namespace_project)   # => "gitlab-ce%1"
   #
   def to_reference(from_project = nil, format: :iid, full: false)
+    return if is_group_milestone?
+
     format_reference = milestone_format_reference(format)
     reference = "#{self.class.reference_prefix}#{format_reference}"
 
@@ -152,6 +177,10 @@ class Milestone < ActiveRecord::Base
     id
   end
 
+  def for_display
+    self
+  end
+
   def can_be_closed?
     active? && issues.opened.count.zero?
   end
@@ -164,8 +193,45 @@ class Milestone < ActiveRecord::Base
     write_attribute(:title, sanitize_title(value)) if value.present?
   end
 
+  def safe_title
+    title.to_slug.normalize.to_s
+  end
+
+  def parent
+    group || project
+  end
+
+  def is_group_milestone?
+    group_id.present?
+  end
+
+  def is_project_milestone?
+    project_id.present?
+  end
+
   private
 
+  # Milestone titles must be unique across project milestones and group milestones
+  def uniqueness_of_title
+    if project
+      relation = Milestone.for_projects_and_groups([project_id], [project.group&.id])
+    elsif group
+      project_ids = group.projects.map(&:id)
+      relation = Milestone.for_projects_and_groups(project_ids, [group.id])
+    end
+
+    title_exists = relation.find_by_title(title)
+    errors.add(:title, "already being used for another group or project milestone.") if title_exists
+  end
+
+  # Milestone should be either a project milestone or a group milestone
+  def milestone_type_check
+    if group_id && project_id
+      field = project_id_changed? ? :project_id : :group_id
+      errors.add(field, "milestone should belong either to a project or a group.")
+    end
+  end
+
   def milestone_format_reference(format = :iid)
     raise ArgumentError, 'Unknown format' unless [:iid, :name].include?(format)
 
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index 8dd0846f3bc..a03a7abfeb1 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -2,8 +2,11 @@ class IssuableBaseService < BaseService
   private
 
   def create_milestone_note(issuable)
+    milestone = issuable.milestone
+    return if milestone && milestone.is_group_milestone?
+
     SystemNoteService.change_milestone(
-      issuable, issuable.project, current_user, issuable.milestone)
+      issuable, issuable.project, current_user, milestone)
   end
 
   def create_labels_note(issuable, old_labels)
@@ -89,10 +92,12 @@ class IssuableBaseService < BaseService
     milestone_id = params[:milestone_id]
     return unless milestone_id
 
-    if milestone_id == IssuableFinder::NONE ||
-        project.milestones.find_by(id: milestone_id).nil?
-      params[:milestone_id] = ''
-    end
+    params[:milestone_id] = '' if milestone_id == IssuableFinder::NONE
+
+    milestone =
+      Milestone.for_projects_and_groups([project.id], [project.group&.id]).find_by_id(milestone_id)
+
+    params[:milestone_id] = '' unless milestone
   end
 
   def filter_labels
diff --git a/app/services/issues/move_service.rb b/app/services/issues/move_service.rb
index 711f4035c55..29def25719d 100644
--- a/app/services/issues/move_service.rb
+++ b/app/services/issues/move_service.rb
@@ -61,8 +61,18 @@ module Issues
     end
 
     def cloneable_milestone_id
-      @new_project.milestones
-        .find_by(title: @old_issue.milestone.try(:title)).try(:id)
+      title = @old_issue.milestone&.title
+      return unless title
+
+      if @new_project.group && can?(current_user, :read_group, @new_project.group)
+        group_id = @new_project.group.id
+      end
+
+      params =
+        { title: title, project_ids: @new_project.id, group_ids: group_id }
+
+      milestones = MilestonesFinder.new(params).execute
+      milestones.first&.id
     end
 
     def rewrite_notes
diff --git a/app/services/milestones/base_service.rb b/app/services/milestones/base_service.rb
index 176ab9f1ab5..4963601ea8b 100644
--- a/app/services/milestones/base_service.rb
+++ b/app/services/milestones/base_service.rb
@@ -1,4 +1,10 @@
 module Milestones
   class BaseService < ::BaseService
+    # Parent can either a group or a project
+    attr_accessor :parent, :current_user, :params
+
+    def initialize(parent, user, params = {})
+      @parent, @current_user, @params = parent, user, params.dup
+    end
   end
 end
diff --git a/app/services/milestones/close_service.rb b/app/services/milestones/close_service.rb
index 608fc49d766..776ec4b287b 100644
--- a/app/services/milestones/close_service.rb
+++ b/app/services/milestones/close_service.rb
@@ -1,7 +1,7 @@
 module Milestones
   class CloseService < Milestones::BaseService
     def execute(milestone)
-      if milestone.close
+      if milestone.close && milestone.is_project_milestone?
         event_service.close_milestone(milestone, current_user)
       end
 
diff --git a/app/services/milestones/create_service.rb b/app/services/milestones/create_service.rb
index b8e08c9f1eb..aef3124c7e3 100644
--- a/app/services/milestones/create_service.rb
+++ b/app/services/milestones/create_service.rb
@@ -1,9 +1,9 @@
 module Milestones
   class CreateService < Milestones::BaseService
     def execute
-      milestone = project.milestones.new(params)
+      milestone = parent.milestones.new(params)
 
-      if milestone.save
+      if milestone.save && milestone.is_project_milestone?
         event_service.open_milestone(milestone, current_user)
       end
 
diff --git a/app/services/milestones/reopen_service.rb b/app/services/milestones/reopen_service.rb
index 573f9ee5c21..5b8b682caaf 100644
--- a/app/services/milestones/reopen_service.rb
+++ b/app/services/milestones/reopen_service.rb
@@ -1,7 +1,7 @@
 module Milestones
   class ReopenService < Milestones::BaseService
     def execute(milestone)
-      if milestone.activate
+      if milestone.activate && milestone.is_project_milestone?
         event_service.reopen_milestone(milestone, current_user)
       end
 
diff --git a/app/services/milestones/update_service.rb b/app/services/milestones/update_service.rb
index ed64847f429..31b441ed476 100644
--- a/app/services/milestones/update_service.rb
+++ b/app/services/milestones/update_service.rb
@@ -5,9 +5,9 @@ module Milestones
 
       case state
       when 'activate'
-        Milestones::ReopenService.new(project, current_user, {}).execute(milestone)
+        Milestones::ReopenService.new(parent, current_user, {}).execute(milestone)
       when 'close'
-        Milestones::CloseService.new(project, current_user, {}).execute(milestone)
+        Milestones::CloseService.new(parent, current_user, {}).execute(milestone)
       end
 
       if params.present?
diff --git a/app/views/groups/milestones/_form.html.haml b/app/views/groups/milestones/_form.html.haml
new file mode 100644
index 00000000000..7f450cd9a93
--- /dev/null
+++ b/app/views/groups/milestones/_form.html.haml
@@ -0,0 +1,27 @@
+= form_for [@group, @milestone], html: { class: 'form-horizontal milestone-form common-note-form js-quick-submit js-requires-input' }  do |f|
+  .row
+    = form_errors(@milestone)
+
+    .col-md-6
+      .form-group
+        = f.label :title, "Title", class: "control-label"
+        .col-sm-10
+          = f.text_field :title, maxlength: 255, class: "form-control", required: true, autofocus: true
+      .form-group.milestone-description
+        = f.label :description, "Description", class: "control-label"
+        .col-sm-10
+          = render layout: 'projects/md_preview', locals: { url: '' } do
+            = render 'projects/zen', f: f, attr: :description, classes: 'note-textarea', placeholder: 'Write milestone description...'
+            .clearfix
+            .error-alert
+
+    = render "shared/milestones/form_dates", f: f
+
+  .form-actions
+    - if @milestone.new_record?
+      = f.submit 'Create milestone', class: "btn-create btn"
+      = link_to "Cancel", group_milestones_path(@group), class: "btn btn-cancel"
+    - else
+      = f.submit 'Update milestone', class: "btn-create btn"
+      = link_to "Cancel", group_milestone_path(@group, @milestone), class: "btn btn-cancel"
+
diff --git a/app/views/groups/milestones/_milestone.html.haml b/app/views/groups/milestones/_milestone.html.haml
index 4c4e0a26728..bae8997e24c 100644
--- a/app/views/groups/milestones/_milestone.html.haml
+++ b/app/views/groups/milestones/_milestone.html.haml
@@ -1,5 +1,6 @@
+
 = render 'shared/milestones/milestone',
-          milestone_path: group_milestone_path(@group, milestone.safe_title, title: milestone.title),
+          milestone_path: group_milestone_route(milestone),
           issues_path: issues_group_path(@group, milestone_title: milestone.title),
           merge_requests_path: merge_requests_group_path(@group, milestone_title: milestone.title),
           milestone: milestone
diff --git a/app/views/groups/milestones/edit.html.haml b/app/views/groups/milestones/edit.html.haml
new file mode 100644
index 00000000000..5f6d7d209d0
--- /dev/null
+++ b/app/views/groups/milestones/edit.html.haml
@@ -0,0 +1,7 @@
+- page_title "Milestones"
+- render "header_title"
+
+%h3.page-title
+  Edit Milestone
+
+= render "form"
diff --git a/app/views/groups/milestones/index.html.haml b/app/views/groups/milestones/index.html.haml
index f91bee0b610..6ceb4092307 100644
--- a/app/views/groups/milestones/index.html.haml
+++ b/app/views/groups/milestones/index.html.haml
@@ -9,11 +9,6 @@
       = link_to new_group_milestone_path(@group), class: "btn btn-new" do
         New milestone
 
-.row-content-block
-  Only milestones from
-  %strong= @group.name
-  group are listed here.
-
 .milestones
   %ul.content-list
     - if @milestones.blank?
diff --git a/app/views/groups/milestones/new.html.haml b/app/views/groups/milestones/new.html.haml
index 7c7573862d0..e24844661ee 100644
--- a/app/views/groups/milestones/new.html.haml
+++ b/app/views/groups/milestones/new.html.haml
@@ -4,40 +4,4 @@
 %h3.page-title
   New Milestone
 
-%p.light
-  This will create milestone in every selected project
-%hr
-
-= form_for @milestone, url: group_milestones_path(@group), html: { class: 'form-horizontal milestone-form common-note-form js-quick-submit js-requires-input' }  do |f|
-  .row
-    - if @milestone.errors.any?
-      #error_explanation
-        .alert.alert-danger
-          %ul
-            - @milestone.errors.full_messages.each do |msg|
-              %li
-                = msg
-
-    .col-md-6
-      .form-group
-        = f.label :title, "Title", class: "control-label"
-        .col-sm-10
-          = f.text_field :title, maxlength: 255, class: "form-control", required: true, autofocus: true
-      .form-group.milestone-description
-        = f.label :description, "Description", class: "control-label"
-        .col-sm-10
-          = render layout: 'projects/md_preview', locals: { url: '' } do
-            = render 'projects/zen', f: f, attr: :description, classes: 'note-textarea', placeholder: 'Write milestone description...'
-            .clearfix
-            .error-alert
-      .form-group
-        = f.label :projects, "Projects", class: "control-label"
-        .col-sm-10
-          = f.collection_select :project_ids, @group.projects.non_archived, :id, :name,
-            { selected: @group.projects.non_archived.pluck(:id) }, required: true, multiple: true, class: 'select2'
-
-    = render "shared/milestones/form_dates", f: f
-
-  .form-actions
-    = f.submit 'Create milestone', class: "btn-create btn"
-    = link_to "Cancel", group_milestones_path(@group), class: "btn btn-cancel"
+= render "form"
diff --git a/app/views/groups/milestones/show.html.haml b/app/views/groups/milestones/show.html.haml
index 33e68bc766e..54b1b7a734a 100644
--- a/app/views/groups/milestones/show.html.haml
+++ b/app/views/groups/milestones/show.html.haml
@@ -1,4 +1,4 @@
 = render "header_title"
 = render 'shared/milestones/top', milestone: @milestone, group: @group
-= render 'shared/milestones/tabs', milestone: @milestone, show_project_name: true
+= render 'shared/milestones/tabs', milestone: @milestone, show_project_name: true if @milestone.is_legacy_group_milestone?
 = render 'shared/milestones/sidebar', milestone: @milestone, affix_offset: 102
diff --git a/app/views/shared/milestones/_milestone.html.haml b/app/views/shared/milestones/_milestone.html.haml
index ecc8b42979c..6f6a036b13f 100644
--- a/app/views/shared/milestones/_milestone.html.haml
+++ b/app/views/shared/milestones/_milestone.html.haml
@@ -1,10 +1,15 @@
 - dashboard = local_assigns[:dashboard]
-- custom_dom_id = dom_id(@project ? milestone : milestone.milestones.first)
+- custom_dom_id = dom_id(milestone.try(:milestones) ? milestone.milestones.first : milestone)
 
 %li{ class: "milestone milestone-#{milestone.closed? ? 'closed' : 'open'}", id: custom_dom_id }
   .row
     .col-sm-6
       %strong= link_to truncate(milestone.title, length: 100), milestone_path
+      - if milestone.is_group_milestone?
+        %span - Group Milestone
+      - else
+        %span - Project Milestone
+
     .col-sm-6
       .pull-right.light #{milestone.percent_complete(current_user)}% complete
   .row
@@ -13,26 +18,32 @@
       &middot;
       = link_to pluralize(milestone.merge_requests.size, 'Merge Request'), merge_requests_path
     .col-sm-6= milestone_progress_bar(milestone)
-  - if milestone.is_a?(GlobalMilestone)
+  - if milestone.is_a?(GlobalMilestone) || milestone.is_group_milestone?
     .row
       .col-sm-6
-        .expiration= render('shared/milestone_expired', milestone: milestone)
-        .projects
-          - milestone.milestones.each do |milestone|
-            = link_to milestone_path(milestone) do
-              %span.label.label-gray
-                = dashboard ? milestone.project.name_with_namespace : milestone.project.name
+        - if milestone.is_legacy_group_milestone?
+          .expiration= render('shared/milestone_expired', milestone: milestone)
+          .projects
+            - milestone.milestones.each do |milestone|
+              = link_to milestone_path(milestone) do
+                %span.label.label-gray
+                  = dashboard ? milestone.project.name_with_namespace : milestone.project.name
       - if @group
-        .col-sm-6
+        .col-sm-6.milestone-actions
           - if can?(current_user, :admin_milestones, @group)
+            - if milestone.is_group_milestone?
+              = link_to edit_group_milestone_path(@group, milestone.id), class: "btn btn-xs btn-grouped" do
+                Edit
+              \
             - if milestone.closed?
-              = link_to 'Reopen Milestone', group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: {state_event: :activate }), method: :put, class: "btn btn-xs btn-grouped btn-reopen"
+              = link_to 'Reopen Milestone', group_milestone_route(milestone, {state_event: :activate }), method: :put, class: "btn btn-xs btn-grouped btn-reopen"
             - else
-              = link_to 'Close Milestone', group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: {state_event: :close }), method: :put, class: "btn btn-xs btn-close"
+              = link_to 'Close Milestone', group_milestone_route(milestone, {state_event: :close }), method: :put, class: "btn btn-xs btn-grouped btn-close"
 
   - if @project
     .row
-      .col-sm-6= render('shared/milestone_expired', milestone: milestone)
+      .col-sm-6
+        = render('shared/milestone_expired', milestone: milestone)
       .col-sm-6.milestone-actions
         - if can?(current_user, :admin_milestone, milestone.project) and milestone.active?
           = link_to edit_project_milestone_path(milestone.project, milestone), class: "btn btn-xs btn-grouped" do
diff --git a/app/views/shared/milestones/_top.html.haml b/app/views/shared/milestones/_top.html.haml
index 20a12613cfc..b93837e3087 100644
--- a/app/views/shared/milestones/_top.html.haml
+++ b/app/views/shared/milestones/_top.html.haml
@@ -22,39 +22,55 @@
   - if group
     .pull-right
       - if can?(current_user, :admin_milestones, group)
+        - if milestone.is_group_milestone?
+          = link_to edit_group_milestone_path(group, milestone.iid), class: "btn btn btn-grouped" do
+            Edit
         - if milestone.active?
-          = link_to 'Close Milestone', group_milestone_path(group, milestone.safe_title, title: milestone.title, milestone: {state_event: :close }), method: :put, class: "btn btn-grouped btn-close"
+          = link_to 'Close Milestone', group_milestone_route(milestone, {state_event: :close }), method: :put, class: "btn btn-grouped btn-close"
         - else
-          = link_to 'Reopen Milestone', group_milestone_path(group, milestone.safe_title, title: milestone.title, milestone: {state_event: :activate }), method: :put, class: "btn btn-grouped btn-reopen"
+          = link_to 'Reopen Milestone', group_milestone_route(milestone, {state_event: :activate }), method: :put, class: "btn btn-grouped btn-reopen"
 
 .detail-page-description.milestone-detail
   %h2.title
     = markdown_field(milestone, :title)
+  - if @milestone.is_group_milestone? && @milestone.description.present?
+    %div
+      .description
+        .wiki
+          = markdown_field(@milestone, :description)
 
 - if milestone.complete?(current_user) && milestone.active?
   .alert.alert-success.prepend-top-default
     - close_msg = group ? 'You may close the milestone now.' : 'Navigate to the project to close the milestone.'
     %span All issues for this milestone are closed. #{close_msg}
 
-.table-holder
-  %table.table
-    %thead
-      %tr
-        %th Project
-        %th Open issues
-        %th State
-        %th Due date
-    - milestone.milestones.each do |ms|
-      %tr
-        %td
-          - project_name = group ? ms.project.name : ms.project.name_with_namespace
-          = link_to project_name, project_milestone_path(ms.project, ms)
-        %td
-          = ms.issues_visible_to_user(current_user).opened.count
-        %td
-          - if ms.closed?
-            Closed
-          - else
-            Open
-        %td
-          = ms.expires_at
+- if @milestone.is_legacy_group_milestone? || @milestone.is_dashboard_milestone?
+  .table-holder
+    %table.table
+      %thead
+        %tr
+          %th Project
+          %th Open issues
+          %th State
+          %th Due date
+      - milestone.milestones.each do |ms|
+        %tr
+          %td
+            - project_name = group ? ms.project.name : ms.project.name_with_namespace
+            = link_to project_name, project_milestone_path(ms.project, ms)
+          %td
+            = ms.issues_visible_to_user(current_user).opened.count
+          %td
+            - if ms.closed?
+              Closed
+            - else
+              Open
+          %td
+            = ms.expires_at
+- elsif @milestone.is_group_milestone?
+  %br
+  View
+  = link_to 'Issues', issues_group_path(@group, milestone_title: milestone.title)
+  or
+  = link_to 'Merge Requests', merge_requests_group_path(@group, milestone_title: milestone.title)
+  in this milestone
diff --git a/changelogs/unreleased/issue_30126_be.yml b/changelogs/unreleased/issue_30126_be.yml
new file mode 100644
index 00000000000..96bb8d9574b
--- /dev/null
+++ b/changelogs/unreleased/issue_30126_be.yml
@@ -0,0 +1,4 @@
+---
+title: Add native group milestones
+merge_request:
+author:
diff --git a/config/routes/group.rb b/config/routes/group.rb
index e578dd8b082..23052a6c6dc 100644
--- a/config/routes/group.rb
+++ b/config/routes/group.rb
@@ -12,7 +12,7 @@ scope(path: 'groups/*group_id',
   end
 
   resource :avatar, only: [:destroy]
-  resources :milestones, constraints: { id: /[^\/]+/ }, only: [:index, :show, :update, :new, :create] do
+  resources :milestones, constraints: { id: /[^\/]+/ }, only: [:index, :show, :edit, :update, :new, :create] do
     member do
       get :merge_requests
       get :participants
diff --git a/db/migrate/20170723183807_add_group_id_to_milestones.rb b/db/migrate/20170723183807_add_group_id_to_milestones.rb
new file mode 100644
index 00000000000..e46fc4f80f0
--- /dev/null
+++ b/db/migrate/20170723183807_add_group_id_to_milestones.rb
@@ -0,0 +1,18 @@
+class AddGroupIdToMilestones < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    change_column_null :milestones, :project_id, true
+
+    add_column :milestones, :group_id, :integer
+  end
+
+  def down
+    # We cannot rollback project_id not null constraint if there are records
+    # with null values.
+    execute "DELETE from milestones WHERE project_id IS NULL"
+
+    remove_column :milestones, :group_id
+    change_column :milestones, :project_id, :integer, null: false
+  end
+end
diff --git a/db/migrate/20170724184243_add_group_milestone_id_indexes.rb b/db/migrate/20170724184243_add_group_milestone_id_indexes.rb
new file mode 100644
index 00000000000..d48b1884179
--- /dev/null
+++ b/db/migrate/20170724184243_add_group_milestone_id_indexes.rb
@@ -0,0 +1,19 @@
+class AddGroupMilestoneIdIndexes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  def up
+    add_concurrent_foreign_key :milestones, :namespaces, column: :group_id, on_delete: :cascade
+
+    add_concurrent_index :milestones, :group_id
+  end
+
+  def down
+    remove_foreign_key :milestones, column: :group_id
+
+    remove_concurrent_index :milestones, :group_id
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index f4d83a4dd9c..d50e623d0f0 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170703102400) do
+ActiveRecord::Schema.define(version: 20170724184243) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -829,7 +829,7 @@ ActiveRecord::Schema.define(version: 20170703102400) do
 
   create_table "milestones", force: :cascade do |t|
     t.string "title", null: false
-    t.integer "project_id", null: false
+    t.integer "project_id"
     t.text "description"
     t.date "due_date"
     t.datetime "created_at"
@@ -840,10 +840,12 @@ ActiveRecord::Schema.define(version: 20170703102400) do
     t.text "description_html"
     t.date "start_date"
     t.integer "cached_markdown_version"
+    t.integer "group_id"
   end
 
   add_index "milestones", ["description"], name: "index_milestones_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
   add_index "milestones", ["due_date"], name: "index_milestones_on_due_date", using: :btree
+  add_index "milestones", ["group_id"], name: "index_milestones_on_group_id", using: :btree
   add_index "milestones", ["project_id", "iid"], name: "index_milestones_on_project_id_and_iid", unique: true, using: :btree
   add_index "milestones", ["title"], name: "index_milestones_on_title", using: :btree
   add_index "milestones", ["title"], name: "index_milestones_on_title_trigram", using: :gin, opclasses: {"title"=>"gin_trgm_ops"}
@@ -1601,6 +1603,7 @@ ActiveRecord::Schema.define(version: 20170703102400) do
   add_foreign_key "merge_requests", "projects", column: "target_project_id", name: "fk_a6963e8447", on_delete: :cascade
   add_foreign_key "merge_requests_closing_issues", "issues", on_delete: :cascade
   add_foreign_key "merge_requests_closing_issues", "merge_requests", on_delete: :cascade
+  add_foreign_key "milestones", "namespaces", column: "group_id", name: "fk_95650a40d4", on_delete: :cascade
   add_foreign_key "milestones", "projects", name: "fk_9bd0a0c791", on_delete: :cascade
   add_foreign_key "notes", "projects", name: "fk_99e097b079", on_delete: :cascade
   add_foreign_key "oauth_openid_requests", "oauth_access_grants", column: "access_grant_id", name: "fk_oauth_openid_requests_oauth_access_grants_access_grant_id"
diff --git a/doc/user/project/milestones/index.md b/doc/user/project/milestones/index.md
index 99233ed5ae2..1848514e2dd 100644
--- a/doc/user/project/milestones/index.md
+++ b/doc/user/project/milestones/index.md
@@ -21,14 +21,11 @@ Once you fill in all the details, hit the **Create milestone** button.
 >**Note:**
 You need [Master permissions](../../permissions.md) in order to create a milestone.
 
-You can create a milestone for several projects in the same group simultaneously.
-On the group's **Issues ➔ Milestones** page, you will be able to see the status
-of that milestone across all of the selected projects. To create a new milestone
-for selected projects in the group, click the **New milestone** button. The
-form is the same as when creating a milestone for a specific project with the
-addition of the selection of the projects you want to inherit this milestone.
+You can create a milestone for a group that will be shared across group projects.
+On the group's **Issues ➔ Milestones** page, you will be able to see the state
+of that milestone and the issues/merge requests count that it shares across the group projects. To create a new milestone click the **New milestone** button. The form is the same as when creating a milestone for a specific project which you can find in the previous item.
 
-![Creating a group milestone](img/milestone_group_create.png)
+In addition to that you will be able to filter issues or merge requests by group milestones in all projects that belongs to the milestone group.
 
 ## Special milestone filters
 
diff --git a/features/group/milestones.feature b/features/group/milestones.feature
index 1c1539b3e12..2211acfee20 100644
--- a/features/group/milestones.feature
+++ b/features/group/milestones.feature
@@ -22,12 +22,12 @@ Feature: Group Milestones
     Then I should see group milestone with descriptions and expiry date
     And I should see group milestone with all issues and MRs assigned to that milestone
 
-  Scenario: Create multiple milestones with one form
+  Scenario: Create group milestones
     Given I visit group "Owned" milestones page
     And I click new milestone button
     And I fill milestone name
     When I press create mileston button
-    Then milestone in each project should be created
+    Then group milestone should be created
 
   Scenario: I should see Issues listed with labels
     Given Group has projects with milestones
diff --git a/features/steps/group/milestones.rb b/features/steps/group/milestones.rb
index 7288dc87005..915d766dd60 100644
--- a/features/steps/group/milestones.rb
+++ b/features/steps/group/milestones.rb
@@ -54,14 +54,9 @@ class Spinach::Features::GroupMilestones < Spinach::FeatureSteps
     click_button "Create milestone"
   end
 
-  step 'milestone in each project should be created' do
+  step 'group milestone should be created' do
     group = Group.find_by(name: 'Owned')
-    expect(page).to have_content "Milestone v2.9.0"
-    expect(group.projects).to be_present
-
-    group.projects.each do |project|
-      expect(page).to have_content project.name
-    end
+    expect(page).to have_content group.milestones.find_by_title('v2.9.0').title
   end
 
   step 'I should see the "bug" label' do
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 94168fa4ebc..fdc0c562248 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -255,7 +255,7 @@ module API
 
     class ProjectEntity < Grape::Entity
       expose :id, :iid
-      expose(:project_id) { |entity| entity.project.id }
+      expose(:project_id) { |entity| entity&.project.try(:id) }
       expose :title, :description
       expose :state, :created_at, :updated_at
     end
@@ -267,7 +267,12 @@ module API
       expose :deleted_file?, as: :deleted_file
     end
 
-    class Milestone < ProjectEntity
+    class Milestone < Grape::Entity
+      expose :id, :iid
+      expose(:project_id) { |entity| entity&.project_id }
+      expose(:group_id) { |entity| entity&.group_id }
+      expose :title, :description
+      expose :state, :created_at, :updated_at
       expose :due_date
       expose :start_date
     end
diff --git a/spec/controllers/groups/milestones_controller_spec.rb b/spec/controllers/groups/milestones_controller_spec.rb
index c6e5fb61cf9..aad67dd0164 100644
--- a/spec/controllers/groups/milestones_controller_spec.rb
+++ b/spec/controllers/groups/milestones_controller_spec.rb
@@ -2,8 +2,8 @@ require 'spec_helper'
 
 describe Groups::MilestonesController do
   let(:group) { create(:group) }
-  let(:project) { create(:empty_project, group: group) }
-  let(:project2) { create(:empty_project, group: group) }
+  let!(:project) { create(:empty_project, group: group) }
+  let!(:project2) { create(:empty_project, group: group) }
   let(:user)    { create(:user) }
   let(:title) { '肯定不是中文的问题' }
   let(:milestone) do
@@ -17,24 +17,67 @@ describe Groups::MilestonesController do
   end
   let(:milestone_path) { group_milestone_path(group, milestone.safe_title, title: milestone.title) }
 
+  let(:milestone_params) do
+    {
+      title: title,
+      start_date: Date.today,
+      due_date: 1.month.from_now.to_date
+    }
+  end
+
   before do
     sign_in(user)
     group.add_owner(user)
     project.team << [user, :master]
   end
 
-  describe "#index" do
+  describe '#index' do
     it 'shows group milestones page' do
       get :index, group_id: group.to_param
 
       expect(response).to have_http_status(200)
     end
 
-    it 'shows group milestones JSON' do
-      get :index, group_id: group.to_param, format: :json
+    context 'as JSON' do
+      let!(:milestone) { create(:milestone, group: group, title: 'group milestone') }
+      let!(:legacy_milestone1) { create(:milestone, project: project, title: 'legacy') }
+      let!(:legacy_milestone2) { create(:milestone, project: project2, title: 'legacy') }
 
-      expect(response).to have_http_status(200)
-      expect(response.content_type).to eq 'application/json'
+      it 'lists legacy group milestones and group milestones' do
+        get :index, group_id: group.to_param, format: :json
+
+        milestones = JSON.parse(response.body)
+
+        expect(milestones.count).to eq(2)
+        expect(milestones.first["title"]).to eq("group milestone")
+        expect(milestones.second["title"]).to eq("legacy")
+        expect(response).to have_http_status(200)
+        expect(response.content_type).to eq 'application/json'
+      end
+    end
+  end
+
+  describe '#show' do
+    let(:milestone1) { create(:milestone, project: project, title: 'legacy') }
+    let(:milestone2) { create(:milestone, project: project, title: 'legacy') }
+    let(:group_milestone) { create(:milestone, group: group) }
+
+    context 'when there is a title parameter' do
+      it 'searchs for a legacy group milestone' do
+        expect(GlobalMilestone).to receive(:build)
+        expect(Milestone).not_to receive(:find_by_iid)
+
+        get :show, group_id: group.to_param, id: title, title: milestone1.safe_title
+      end
+    end
+
+    context 'when there is not a title parameter' do
+      it 'searchs for a group milestone' do
+        expect(GlobalMilestone).not_to receive(:build)
+        expect(Milestone).to receive(:find_by_iid)
+
+        get :show, group_id: group.to_param, id: group_milestone.id
+      end
     end
   end
 
@@ -44,16 +87,57 @@ describe Groups::MilestonesController do
     it "creates group milestone with Chinese title" do
       post :create,
            group_id: group.to_param,
-           milestone: { project_ids: [project.id, project2.id], title: title }
+           milestone: milestone_params
 
-      expect(response).to redirect_to(group_milestone_path(group, title.to_slug.to_s, title: title))
-      expect(Milestone.where(title: title).count).to eq(2)
+      milestone = Milestone.find_by_title(title)
+
+      expect(response).to redirect_to(group_milestone_path(group, milestone.iid))
+      expect(milestone.group_id).to eq(group.id)
+      expect(milestone.due_date).to eq(milestone_params[:due_date])
+      expect(milestone.start_date).to eq(milestone_params[:start_date])
+    end
+  end
+
+  describe "#update" do
+    let(:milestone) { create(:milestone, group: group) }
+
+    it "updates group milestone" do
+      milestone_params[:title] = "title changed"
+
+      put :update,
+           id: milestone.iid,
+           group_id: group.to_param,
+           milestone: milestone_params
+
+      milestone.reload
+      expect(response).to redirect_to(group_milestone_path(group, milestone.iid))
+      expect(milestone.title).to eq("title changed")
     end
 
-    it "redirects to new when there are no project ids" do
-      post :create, group_id: group.to_param, milestone: { title: title, project_ids: [""] }
-      expect(response).to render_template :new
-      expect(assigns(:milestone).errors).not_to be_nil
+    context "legacy group milestones" do
+      let!(:milestone1) { create(:milestone, project: project, title: 'legacy milestone', description: "old description") }
+      let!(:milestone2) { create(:milestone, project: project2, title: 'legacy milestone', description: "old description") }
+
+      it "updates only group milestones state" do
+        milestone_params[:title] = "title changed"
+        milestone_params[:description] = "description changed"
+        milestone_params[:state_event] = "close"
+
+        put :update,
+             id: milestone1.title.to_slug.to_s,
+             group_id: group.to_param,
+             milestone: milestone_params,
+             title: milestone1.title
+
+        expect(response).to redirect_to(group_milestone_path(group, milestone1.safe_title, title: milestone1.title))
+
+        [milestone1, milestone2].each do |milestone|
+          milestone.reload
+          expect(milestone.title).to eq("legacy milestone")
+          expect(milestone.description).to eq("old description")
+          expect(milestone.state).to eq("closed")
+        end
+      end
     end
   end
 
@@ -156,7 +240,7 @@ describe Groups::MilestonesController do
       it 'does not 404' do
         post :create,
              group_id: group.to_param,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).not_to have_http_status(404)
       end
@@ -164,7 +248,7 @@ describe Groups::MilestonesController do
       it 'does not redirect to the correct casing' do
         post :create,
              group_id: group.to_param,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).not_to have_http_status(301)
       end
@@ -176,7 +260,7 @@ describe Groups::MilestonesController do
       it 'returns not found' do
         post :create,
              group_id: redirect_route.path,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).to have_http_status(404)
       end
diff --git a/spec/controllers/projects/milestones_controller_spec.rb b/spec/controllers/projects/milestones_controller_spec.rb
index 84a61b2784e..bb5a340cd96 100644
--- a/spec/controllers/projects/milestones_controller_spec.rb
+++ b/spec/controllers/projects/milestones_controller_spec.rb
@@ -31,6 +31,40 @@ describe Projects::MilestonesController do
     end
   end
 
+  describe "#index" do
+    context "as html" do
+      before do
+        get :index, namespace_id: project.namespace.id, project_id: project.id
+      end
+
+      it "queries only projects milestones" do
+        milestones = assigns(:milestones)
+
+        expect(milestones.count).to eq(1)
+        expect(milestones.where(project_id: nil)).to be_empty
+      end
+    end
+
+    context "as json" do
+      let!(:group) { create(:group, :public) }
+      let!(:group_milestone) { create(:milestone, group: group) }
+      let!(:group_member) { create(:group_member, group: group, user: user) }
+
+      before do
+        project.update(namespace: group)
+        get :index, namespace_id: project.namespace.id, project_id: project.id, format: :json
+      end
+
+      it "queries projects milestones and groups milestones" do
+        milestones = assigns(:milestones)
+
+        expect(milestones.count).to eq(2)
+        expect(milestones.where(project_id: nil).first).to eq(group_milestone)
+        expect(milestones.where(group_id: nil).first).to eq(milestone)
+      end
+    end
+  end
+
   describe "#destroy" do
     it "removes milestone" do
       expect(issue.milestone_id).to eq(milestone.id)
diff --git a/spec/factories/milestones.rb b/spec/factories/milestones.rb
index 841ab3c73b8..113665ff11b 100644
--- a/spec/factories/milestones.rb
+++ b/spec/factories/milestones.rb
@@ -1,7 +1,13 @@
 FactoryGirl.define do
   factory :milestone do
     title
-    project factory: :empty_project
+
+    transient do
+      project nil
+      group nil
+      project_id nil
+      group_id nil
+    end
 
     trait :active do
       state "active"
@@ -11,6 +17,20 @@ FactoryGirl.define do
       state "closed"
     end
 
+    after(:build) do |milestone, evaluator|
+      if evaluator.group
+        milestone.group = evaluator.group
+      elsif evaluator.group_id
+        milestone.group_id = evaluator.group_id
+      elsif evaluator.project
+        milestone.project = evaluator.project
+      elsif evaluator.project_id
+        milestone.project_id = evaluator.project_id
+      else
+        milestone.project = create(:empty_project)
+      end
+    end
+
     factory :active_milestone, traits: [:active]
     factory :closed_milestone, traits: [:closed]
   end
diff --git a/spec/features/groups/milestone_spec.rb b/spec/features/groups/milestone_spec.rb
index 330310eae6b..9b6eb946f4b 100644
--- a/spec/features/groups/milestone_spec.rb
+++ b/spec/features/groups/milestone_spec.rb
@@ -33,4 +33,32 @@ feature 'Group milestones', :feature, :js do
       expect(find('.start_date')).to have_content(Date.today.at_beginning_of_month.strftime('%b %-d, %Y'))
     end
   end
+
+  context 'milestones list' do
+    let!(:other_project) { create(:project_empty_repo, group: group) }
+
+    let!(:active_group_milestone) { create(:milestone, group: group, state: 'active') }
+    let!(:active_project_milestone1) { create(:milestone, project: project, state: 'active', title: 'v1.0') }
+    let!(:active_project_milestone2) { create(:milestone, project: other_project, state: 'active', title: 'v1.0') }
+    let!(:closed_group_milestone) { create(:milestone, group: group, state: 'closed') }
+    let!(:closed_project_milestone1) { create(:milestone, project: project, state: 'closed', title: 'v2.0') }
+    let!(:closed_project_milestone2) { create(:milestone, project: other_project, state: 'closed', title: 'v2.0') }
+
+    before do
+      visit group_milestones_path(group)
+    end
+
+    it 'counts milestones correctly' do
+      expect(find('.top-area .active .badge').text).to eq("2")
+      expect(find('.top-area .closed .badge').text).to eq("2")
+      expect(find('.top-area .all .badge').text).to eq("4")
+    end
+
+    it 'lists legacy group milestones and group milestones' do
+      legacy_milestone = GroupMilestone.build_collection(group, group.projects, { state: 'active' }).first
+
+      expect(page).to have_selector("#milestone_#{active_group_milestone.id}", count: 1)
+      expect(page).to have_selector("#milestone_#{legacy_milestone.milestones.first.id}", count: 1)
+    end
+  end
 end
diff --git a/spec/features/milestone_spec.rb b/spec/features/milestone_spec.rb
index 880c53343bc..2afdd0d321c 100644
--- a/spec/features/milestone_spec.rb
+++ b/spec/features/milestone_spec.rb
@@ -1,10 +1,12 @@
 require 'rails_helper'
 
 feature 'Milestone', feature: true do
-  let(:project) { create(:empty_project, :public) }
+  let(:group) { create(:group, :public) }
+  let(:project) { create(:empty_project, :public, namespace: group) }
   let(:user)   { create(:user) }
 
   before do
+    create(:group_member, group: group, user: user)
     project.team << [user, :master]
     gitlab_sign_in(user)
   end
@@ -37,8 +39,8 @@ feature 'Milestone', feature: true do
     end
   end
 
-  feature 'Open a milestone with an existing title' do
-    scenario 'displays validation message' do
+  feature 'Open a project milestone with an existing title' do
+    scenario 'displays validation message when there is a project milestone with same title' do
       milestone = create(:milestone, project: project, title: 8.7)
 
       visit new_project_milestone_path(project)
@@ -47,7 +49,20 @@ feature 'Milestone', feature: true do
       end
       find('input[name="commit"]').click
 
-      expect(find('.alert-danger')).to have_content('Title has already been taken')
+      expect(find('.alert-danger')).to have_content('already being used for another group or project milestone.')
+    end
+
+    scenario 'displays validation message when there is a group milestone with same title' do
+      milestone = create(:milestone, project_id: nil, group: project.group, title: 8.7)
+
+      visit new_group_milestone_path(project.group)
+
+      page.within '.milestone-form' do
+        fill_in "milestone_title", with: milestone.title
+      end
+      find('input[name="commit"]').click
+
+      expect(find('.alert-danger')).to have_content('already being used for another group or project milestone.')
     end
   end
 end
diff --git a/spec/finders/issues_finder_spec.rb b/spec/finders/issues_finder_spec.rb
index 4a52f0d5c58..bef4fd44331 100644
--- a/spec/finders/issues_finder_spec.rb
+++ b/spec/finders/issues_finder_spec.rb
@@ -59,6 +59,23 @@ describe IssuesFinder do
         end
       end
 
+      context 'filtering by group milestone' do
+        let!(:group) { create(:group, :public) }
+        let(:group_milestone) { create(:milestone, group: group) }
+        let!(:group_member) { create(:group_member, group: group, user: user) }
+        let(:params) { { milestone_title: group_milestone.title } }
+
+        before do
+          project2.update(namespace: group)
+          issue2.update(milestone: group_milestone)
+          issue3.update(milestone: group_milestone)
+        end
+
+        it 'returns issues assigned to that group milestone' do
+          expect(issues).to contain_exactly(issue2, issue3)
+        end
+      end
+
       context 'filtering by no milestone' do
         let(:params) { { milestone_title: Milestone::None.title } }
 
diff --git a/spec/finders/merge_requests_finder_spec.rb b/spec/finders/merge_requests_finder_spec.rb
index 5eb26de6c92..b46218bf72e 100644
--- a/spec/finders/merge_requests_finder_spec.rb
+++ b/spec/finders/merge_requests_finder_spec.rb
@@ -47,6 +47,25 @@ describe MergeRequestsFinder do
       expect(merge_requests).to contain_exactly(merge_request1)
     end
 
+    context 'filtering by group milestone' do
+      let!(:group) { create(:group, :public) }
+      let(:group_milestone) { create(:milestone, group: group) }
+      let!(:group_member) { create(:group_member, group: group, user: user) }
+      let(:params) { { milestone_title: group_milestone.title } }
+
+      before do
+        project2.update(namespace: group)
+        merge_request2.update(milestone: group_milestone)
+        merge_request3.update(milestone: group_milestone)
+      end
+
+      it 'returns issues assigned to that group milestone' do
+        merge_requests = described_class.new(user, params).execute
+
+        expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+      end
+    end
+
     context 'with created_after and created_before params' do
       let(:project4) { create(:empty_project, forked_from_project: project1) }
 
diff --git a/spec/finders/milestones_finder_spec.rb b/spec/finders/milestones_finder_spec.rb
new file mode 100644
index 00000000000..32ec983c5b8
--- /dev/null
+++ b/spec/finders/milestones_finder_spec.rb
@@ -0,0 +1,90 @@
+require 'spec_helper'
+
+describe MilestonesFinder do
+  let(:group) { create(:group) }
+  let(:project_1) { create(:empty_project, namespace: group) }
+  let(:project_2) { create(:empty_project, namespace: group) }
+  let!(:milestone_1) { create(:milestone, group: group, title: 'one test', due_date: Date.today) }
+  let!(:milestone_2) { create(:milestone, group: group) }
+  let!(:milestone_3) { create(:milestone, project: project_1, state: 'active', due_date: Date.tomorrow) }
+  let!(:milestone_4) { create(:milestone, project: project_2, state: 'active') }
+
+  it 'it returns milestones for projects' do
+    result = described_class.new(project_ids: [project_1.id, project_2.id], state: 'all').execute
+
+    expect(result).to contain_exactly(milestone_3, milestone_4)
+  end
+
+  it 'returns milestones for groups' do
+    result = described_class.new(group_ids: group.id,  state: 'all').execute
+
+    expect(result).to contain_exactly(milestone_1, milestone_2)
+  end
+
+  it 'returns milestones for groups and projects' do
+    result = described_class.new(project_ids: [project_1.id, project_2.id], group_ids: group.id, state: 'all').execute
+
+    expect(result).to contain_exactly(milestone_1, milestone_2, milestone_3, milestone_4)
+  end
+
+  context 'with filters' do
+    let(:params) do
+      {
+        project_ids: [project_1.id, project_2.id],
+        group_ids: group.id,
+        state: 'all'
+      }
+    end
+
+    before do
+      milestone_1.close
+      milestone_3.close
+    end
+
+    it 'filters by active state' do
+      params[:state] = 'active'
+      result = described_class.new(params).execute
+
+      expect(result).to contain_exactly(milestone_2, milestone_4)
+    end
+
+    it 'filters by closed state' do
+      params[:state] = 'closed'
+      result = described_class.new(params).execute
+
+      expect(result).to contain_exactly(milestone_1, milestone_3)
+    end
+
+    it 'filters by title' do
+      result = described_class.new(params.merge(title: 'one test')).execute
+
+      expect(result.to_a).to contain_exactly(milestone_1)
+    end
+  end
+
+  context 'with order' do
+    let(:params) do
+      {
+        project_ids: [project_1.id, project_2.id],
+        group_ids: group.id,
+        state: 'all'
+      }
+    end
+
+    it "default orders by due date" do
+      result = described_class.new(params).execute
+
+      expect(result.first).to eq(milestone_1)
+      expect(result.second).to eq(milestone_3)
+    end
+
+    it "orders by parameter" do
+      result = described_class.new(params.merge(order: 'id DESC')).execute
+
+      expect(result.first).to eq(milestone_4)
+      expect(result.second).to eq(milestone_3)
+      expect(result.third).to eq(milestone_2)
+      expect(result.fourth).to eq(milestone_1)
+    end
+  end
+end
diff --git a/spec/fixtures/api/schemas/public_api/v3/issues.json b/spec/fixtures/api/schemas/public_api/v3/issues.json
index f2ee9c925ae..51b0822bc66 100644
--- a/spec/fixtures/api/schemas/public_api/v3/issues.json
+++ b/spec/fixtures/api/schemas/public_api/v3/issues.json
@@ -22,7 +22,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
index 01f9fbb2c89..b5c74bcc26e 100644
--- a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
@@ -53,7 +53,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v4/issues.json b/spec/fixtures/api/schemas/public_api/v4/issues.json
index 2d1c84ee93d..bd6bfc03199 100644
--- a/spec/fixtures/api/schemas/public_api/v4/issues.json
+++ b/spec/fixtures/api/schemas/public_api/v4/issues.json
@@ -22,7 +22,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
index 51642e8cbb8..60aa47c1259 100644
--- a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
@@ -53,7 +53,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/lib/gitlab/import_export/all_models.yml b/spec/lib/gitlab/import_export/all_models.yml
index 562f0c2991c..83fe26668cb 100644
--- a/spec/lib/gitlab/import_export/all_models.yml
+++ b/spec/lib/gitlab/import_export/all_models.yml
@@ -45,6 +45,7 @@ label:
 - merge_requests
 - priorities
 milestone:
+- group
 - project
 - issues
 - labels
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index b4a7e956686..4ef3db3721f 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -82,6 +82,7 @@ Milestone:
 - id
 - title
 - project_id
+- group_id
 - description
 - due_date
 - start_date
diff --git a/spec/models/milestone_spec.rb b/spec/models/milestone_spec.rb
index 45953023a36..2649d04bee3 100644
--- a/spec/models/milestone_spec.rb
+++ b/spec/models/milestone_spec.rb
@@ -6,9 +6,6 @@ describe Milestone, models: true do
       allow(subject).to receive(:set_iid).and_return(false)
     end
 
-    it { is_expected.to validate_presence_of(:title) }
-    it { is_expected.to validate_presence_of(:project) }
-
     describe 'start_date' do
       it 'adds an error when start_date is greated then due_date' do
         milestone = build(:milestone, start_date: Date.tomorrow, due_date: Date.yesterday)
@@ -37,17 +34,42 @@ describe Milestone, models: true do
     end
   end
 
-  describe "unique milestone title per project" do
-    it "does not accept the same title in a project twice" do
-      new_milestone = Milestone.new(project: milestone.project, title: milestone.title)
-      expect(new_milestone).not_to be_valid
+  describe "unique milestone title" do
+    context "per project" do
+      it "does not accept the same title in a project twice" do
+        new_milestone = Milestone.new(project: milestone.project, title: milestone.title)
+        expect(new_milestone).not_to be_valid
+      end
+
+      it "accepts the same title in another project" do
+        project = create(:empty_project)
+        new_milestone = Milestone.new(project: project, title: milestone.title)
+
+        expect(new_milestone).to be_valid
+      end
     end
 
-    it "accepts the same title in another project" do
-      project = build(:empty_project)
-      new_milestone = Milestone.new(project: project, title: milestone.title)
+    context "per group" do
+      let(:group) { create(:group) }
+      let(:milestone) { create(:milestone, group: group) }
 
-      expect(new_milestone).to be_valid
+      before do
+        project.update(group: group)
+      end
+
+      it "does not accept the same title in a group twice" do
+        new_milestone = Milestone.new(group: group, title: milestone.title)
+
+        expect(new_milestone).not_to be_valid
+      end
+
+      it "does not accept the same title of a child project milestone" do
+        create(:milestone, project: group.projects.first)
+
+        new_milestone = Milestone.new(group: group, title: milestone.title)
+
+        expect(new_milestone).not_to be_valid
+      end
     end
   end
 
diff --git a/spec/services/issues/move_service_spec.rb b/spec/services/issues/move_service_spec.rb
index d1dd1466d95..36d5038fb95 100644
--- a/spec/services/issues/move_service_spec.rb
+++ b/spec/services/issues/move_service_spec.rb
@@ -37,9 +37,6 @@ describe Issues::MoveService, services: true do
 
   describe '#execute' do
     shared_context 'issue move executed' do
-      let!(:milestone2) do
-        create(:milestone, project_id: new_project.id, title: 'v9.0')
-      end
       let!(:award_emoji) { create(:award_emoji, awardable: old_issue) }
 
       let!(:new_issue) { move_service.execute(old_issue, new_project) }
@@ -48,6 +45,63 @@ describe Issues::MoveService, services: true do
     context 'issue movable' do
       include_context 'user can move issue'
 
+      context 'move to new milestone'  do
+        let(:new_issue) { move_service.execute(old_issue, new_project) }
+
+        context 'project milestone' do
+          let!(:milestone2) do
+            create(:milestone, project_id: new_project.id, title: 'v9.0')
+          end
+
+          it 'assigns milestone to new issue' do
+            expect(new_issue.reload.milestone.title).to eq 'v9.0'
+            expect(new_issue.reload.milestone).to eq(milestone2)
+          end
+        end
+
+        context 'group milestones' do
+          let!(:group) { create(:group, :private) }
+          let!(:group_milestone_1) do
+            create(:milestone, group_id: group.id, title: 'v9.0_group')
+          end
+
+          before do
+            old_issue.update(milestone: group_milestone_1)
+            old_project.update(namespace: group)
+            new_project.update(namespace: group)
+
+            group.add_users([user], GroupMember::DEVELOPER)
+          end
+
+          context 'when moving to a project of the same group' do
+            it 'keeps the same group milestone' do
+              expect(new_issue.reload.project).to eq(new_project)
+              expect(new_issue.reload.milestone).to eq(group_milestone_1)
+            end
+          end
+
+          context 'when moving to a project of a different group' do
+            let!(:group_2) { create(:group, :private) }
+
+            let!(:group_milestone_2) do
+              create(:milestone, group_id: group_2.id, title: 'v9.0_group')
+            end
+
+            before do
+              old_issue.update(milestone: group_milestone_1)
+              new_project.update(namespace: group_2)
+
+              group_2.add_users([user], GroupMember::DEVELOPER)
+            end
+
+            it 'assigns to new group milestone of same title' do
+              expect(new_issue.reload.project).to eq(new_project)
+              expect(new_issue.reload.milestone).to eq(group_milestone_2)
+            end
+          end
+        end
+      end
+
       context 'generic issue' do
         include_context 'issue move executed'
 
@@ -55,11 +109,6 @@ describe Issues::MoveService, services: true do
           expect(new_issue.project).to eq new_project
         end
 
-        it 'assigns milestone to new issue' do
-          expect(new_issue.reload.milestone.title).to eq 'v9.0'
-          expect(new_issue.reload.milestone).to eq(milestone2)
-        end
-
         it 'assign labels to new issue' do
           expected_label_titles = new_issue.reload.labels.map(&:title)
           expect(expected_label_titles).to include 'label1'
diff --git a/spec/services/issues/update_service_spec.rb b/spec/services/issues/update_service_spec.rb
index c26642f5015..d0b991f19ab 100644
--- a/spec/services/issues/update_service_spec.rb
+++ b/spec/services/issues/update_service_spec.rb
@@ -253,13 +253,13 @@ describe Issues::UpdateService, services: true do
       end
 
       context 'when the milestone change' do
-        before do
-          update_issue(milestone: create(:milestone))
-        end
-
         it 'marks todos as done' do
+          update_issue(milestone: create(:milestone))
+
           expect(todo.reload.done?).to eq true
         end
+
+        it_behaves_like 'system notes for milestones'
       end
 
       context 'when the labels change' do
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index ec15b5cac14..be62584ec0e 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -296,13 +296,13 @@ describe MergeRequests::UpdateService, services: true do
       end
 
       context 'when the milestone change' do
-        before do
-          update_merge_request({ milestone: create(:milestone) })
-        end
-
         it 'marks pending todos as done' do
+          update_merge_request({ milestone: create(:milestone) })
+
           expect(pending_todo.reload).to be_done
         end
+
+        it_behaves_like 'system notes for milestones'
       end
 
       context 'when the labels change' do
diff --git a/spec/support/issuable_shared_examples.rb b/spec/support/issuable_shared_examples.rb
index 03011535351..970fe10db2b 100644
--- a/spec/support/issuable_shared_examples.rb
+++ b/spec/support/issuable_shared_examples.rb
@@ -5,3 +5,34 @@ shared_examples 'cache counters invalidator' do
     described_class.new(project, user, {}).execute(merge_request)
   end
 end
+
+shared_examples 'system notes for milestones' do
+  def update_issuable(opts)
+    issuable = try(:issue) || try(:merge_request)
+    described_class.new(project, user, opts).execute(issuable)
+  end
+
+  context 'group milestones' do
+    let(:group) { create(:group) }
+    let(:group_milestone) { create(:milestone, group: group) }
+
+    before do
+      project.update(namespace: group)
+      create(:group_member, group: group, user: user)
+    end
+
+    it 'does not create system note' do
+      expect do
+        update_issuable(milestone: group_milestone)
+      end.not_to change { Note.system.count }
+    end
+  end
+
+  context 'project milestones' do
+    it 'creates system note' do
+      expect do
+        update_issuable(milestone: create(:milestone))
+      end.to change { Note.system.count }.by(1)
+    end
+  end
+end