diff --git a/app/models/project_authorization.rb b/app/models/project_authorization.rb
index a00d43773d9..4c7f4f5a429 100644
--- a/app/models/project_authorization.rb
+++ b/app/models/project_authorization.rb
@@ -5,4 +5,17 @@ class ProjectAuthorization < ActiveRecord::Base
   validates :project, presence: true
   validates :access_level, inclusion: { in: Gitlab::Access.all_values }, presence: true
   validates :user, uniqueness: { scope: [:project, :access_level] }, presence: true
+
+  def self.insert_authorizations(rows, per_batch = 1000)
+    rows.each_slice(per_batch) do |slice|
+      tuples = slice.map do |tuple|
+        tuple.map { |value| connection.quote(value) }
+      end
+
+      connection.execute <<-EOF.strip_heredoc
+      INSERT INTO project_authorizations (user_id, project_id, access_level)
+      VALUES #{tuples.map { |tuple| "(#{tuple.join(', ')})" }.join(', ')}
+      EOF
+    end
+  end
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index 3a17c98eff6..9f5cc149361 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -443,22 +443,16 @@ class User < ActiveRecord::Base
   end
 
   def refresh_authorized_projects
-    transaction do
-      project_authorizations.delete_all
+    Users::RefreshAuthorizedProjectsService.new(self).execute
+  end
 
-      # project_authorizations_union can return multiple records for the same
-      # project/user with different access_level so we take row with the maximum
-      # access_level
-      project_authorizations.connection.execute <<-SQL
-      INSERT INTO project_authorizations (user_id, project_id, access_level)
-      SELECT user_id, project_id, MAX(access_level) AS access_level
-      FROM (#{project_authorizations_union.to_sql}) sub
-      GROUP BY user_id, project_id
-      SQL
+  def remove_project_authorizations(project_ids)
+    project_authorizations.where(id: project_ids).delete_all
+  end
 
-      unless authorized_projects_populated
-        update_column(:authorized_projects_populated, true)
-      end
+  def set_authorized_projects_column
+    unless authorized_projects_populated
+      update_column(:authorized_projects_populated, true)
     end
   end
 
@@ -905,18 +899,6 @@ class User < ActiveRecord::Base
 
   private
 
-  # Returns a union query of projects that the user is authorized to access
-  def project_authorizations_union
-    relations = [
-      personal_projects.select("#{id} AS user_id, projects.id AS project_id, #{Gitlab::Access::MASTER} AS access_level"),
-      groups_projects.select_for_project_authorization,
-      projects.select_for_project_authorization,
-      groups.joins(:shared_projects).select_for_project_authorization
-    ]
-
-    Gitlab::SQL::Union.new(relations)
-  end
-
   def ci_projects_union
     scope  = { access_level: [Gitlab::Access::MASTER, Gitlab::Access::OWNER] }
     groups = groups_projects.where(members: scope)
diff --git a/app/services/users/refresh_authorized_projects_service.rb b/app/services/users/refresh_authorized_projects_service.rb
new file mode 100644
index 00000000000..7d38ac3a374
--- /dev/null
+++ b/app/services/users/refresh_authorized_projects_service.rb
@@ -0,0 +1,128 @@
+module Users
+  # Service for refreshing the authorized projects of a user.
+  #
+  # This particular service class can not be used to update data for the same
+  # user concurrently. Doing so could lead to an incorrect state. To ensure this
+  # doesn't happen a caller must synchronize access (e.g. using
+  # `Gitlab::ExclusiveLease`).
+  #
+  # Usage:
+  #
+  #     user = User.find_by(username: 'alice')
+  #     service = Users::RefreshAuthorizedProjectsService.new(some_user)
+  #     service.execute
+  class RefreshAuthorizedProjectsService
+    attr_reader :user
+
+    LEASE_TIMEOUT = 1.minute.to_i
+
+    # user - The User for which to refresh the authorized projects.
+    def initialize(user)
+      @user = user
+
+      # We need an up to date User object that has access to all relations that
+      # may have been created earlier. The only way to ensure this is to reload
+      # the User object.
+      user.reload
+    end
+
+    # This method returns the updated User object.
+    def execute
+      current = current_authorizations_per_project
+      fresh = fresh_access_levels_per_project
+
+      remove = current.each_with_object([]) do |(project_id, row), array|
+        # rows not in the new list or with a different access level should be
+        # removed.
+        if !fresh[project_id] || fresh[project_id] != row.access_level
+          array << row.id
+        end
+      end
+
+      add = fresh.each_with_object([]) do |(project_id, level), array|
+        # rows not in the old list or with a different access level should be
+        # added.
+        if !current[project_id] || current[project_id].access_level != level
+          array << [user.id, project_id, level]
+        end
+      end
+
+      update_with_lease(remove, add)
+    end
+
+    # Updates the list of authorizations using an exclusive lease.
+    def update_with_lease(remove = [], add = [])
+      lease_key = "refresh_authorized_projects:#{user.id}"
+      lease = Gitlab::ExclusiveLease.new(lease_key, timeout: LEASE_TIMEOUT)
+
+      until uuid = lease.try_obtain
+        # Keep trying until we obtain the lease. If we don't do so we may end up
+        # not updating the list of authorized projects properly. To prevent
+        # hammering Redis too much we'll wait for a bit between retries.
+        sleep(1)
+      end
+
+      begin
+        update_authorizations(remove, add)
+      ensure
+        Gitlab::ExclusiveLease.cancel(lease_key, uuid)
+      end
+    end
+
+    # Updates the list of authorizations for the current user.
+    #
+    # remove - The IDs of the authorization rows to remove.
+    # add - Rows to insert in the form `[user id, project id, access level]`
+    def update_authorizations(remove = [], add = [])
+      return if remove.empty? && add.empty?
+
+      User.transaction do
+        user.remove_project_authorizations(remove) unless remove.empty?
+        ProjectAuthorization.insert_authorizations(add) unless add.empty?
+        user.set_authorized_projects_column
+      end
+
+      # Since we batch insert authorization rows, Rails' associations may get
+      # out of sync. As such we force a reload of the User object.
+      user.reload
+    end
+
+    def fresh_access_levels_per_project
+      fresh_authorizations.each_with_object({}) do |row, hash|
+        hash[row.project_id] = row.access_level
+      end
+    end
+
+    def current_authorizations_per_project
+      current_authorizations.each_with_object({}) do |row, hash|
+        hash[row.project_id] = row
+      end
+    end
+
+    def current_authorizations
+      user.project_authorizations.select(:id, :project_id, :access_level)
+    end
+
+    def fresh_authorizations
+      ProjectAuthorization.
+        unscoped.
+        select('project_id, MAX(access_level) AS access_level').
+        from("(#{project_authorizations_union.to_sql}) #{ProjectAuthorization.table_name}").
+        group(:project_id)
+    end
+
+    private
+
+    # Returns a union query of projects that the user is authorized to access
+    def project_authorizations_union
+      relations = [
+        user.personal_projects.select("#{user.id} AS user_id, projects.id AS project_id, #{Gitlab::Access::MASTER} AS access_level"),
+        user.groups_projects.select_for_project_authorization,
+        user.projects.select_for_project_authorization,
+        user.groups.joins(:shared_projects).select_for_project_authorization
+      ]
+
+      Gitlab::SQL::Union.new(relations)
+    end
+  end
+end
diff --git a/app/workers/authorized_projects_worker.rb b/app/workers/authorized_projects_worker.rb
index fccddb70d18..2badd0680fb 100644
--- a/app/workers/authorized_projects_worker.rb
+++ b/app/workers/authorized_projects_worker.rb
@@ -2,8 +2,6 @@ class AuthorizedProjectsWorker
   include Sidekiq::Worker
   include DedicatedSidekiqQueue
 
-  LEASE_TIMEOUT = 1.minute.to_i
-
   def self.bulk_perform_async(args_list)
     Sidekiq::Client.push_bulk('class' => self, 'args' => args_list)
   end
@@ -11,24 +9,6 @@ class AuthorizedProjectsWorker
   def perform(user_id)
     user = User.find_by(id: user_id)
 
-    refresh(user) if user
-  end
-
-  def refresh(user)
-    lease_key = "refresh_authorized_projects:#{user.id}"
-    lease = Gitlab::ExclusiveLease.new(lease_key, timeout: LEASE_TIMEOUT)
-
-    until uuid = lease.try_obtain
-      # Keep trying until we obtain the lease. If we don't do so we may end up
-      # not updating the list of authorized projects properly. To prevent
-      # hammering Redis too much we'll wait for a bit between retries.
-      sleep(1)
-    end
-
-    begin
-      user.refresh_authorized_projects
-    ensure
-      Gitlab::ExclusiveLease.cancel(lease_key, uuid)
-    end
+    user.refresh_authorized_projects if user
   end
 end
diff --git a/spec/models/project_authorization_spec.rb b/spec/models/project_authorization_spec.rb
new file mode 100644
index 00000000000..33ef67f97a7
--- /dev/null
+++ b/spec/models/project_authorization_spec.rb
@@ -0,0 +1,25 @@
+require 'spec_helper'
+
+describe ProjectAuthorization do
+  let(:user) { create(:user) }
+  let(:project1) { create(:empty_project) }
+  let(:project2) { create(:empty_project) }
+
+  describe '.insert_authorizations' do
+    it 'inserts the authorizations' do
+      described_class.
+        insert_authorizations([[user.id, project1.id, Gitlab::Access::MASTER]])
+
+      expect(user.project_authorizations.count).to eq(1)
+    end
+
+    it 'inserts rows in batches' do
+      described_class.insert_authorizations([
+        [user.id, project1.id, Gitlab::Access::MASTER],
+        [user.id, project2.id, Gitlab::Access::MASTER],
+      ], 1)
+
+      expect(user.project_authorizations.count).to eq(2)
+    end
+  end
+end
diff --git a/spec/services/users/refresh_authorized_projects_service_spec.rb b/spec/services/users/refresh_authorized_projects_service_spec.rb
new file mode 100644
index 00000000000..72c8f7cd8ec
--- /dev/null
+++ b/spec/services/users/refresh_authorized_projects_service_spec.rb
@@ -0,0 +1,185 @@
+require 'spec_helper'
+
+describe Users::RefreshAuthorizedProjectsService do
+  let(:project) { create(:empty_project) }
+  let(:user) { project.namespace.owner }
+  let(:service) { described_class.new(user) }
+
+  def create_authorization(project, user, access_level = Gitlab::Access::MASTER)
+    ProjectAuthorization.
+      create!(project: project, user: user, access_level: access_level)
+  end
+
+  describe '#execute' do
+    before do
+      user.project_authorizations.delete_all
+    end
+
+    it 'updates the authorized projects of the user' do
+      project2 = create(:empty_project)
+      to_remove = create_authorization(project2, user)
+
+      expect(service).to receive(:update_with_lease).
+        with([to_remove.id], [[user.id, project.id, Gitlab::Access::MASTER]])
+
+      service.execute
+    end
+
+    it 'sets the access level of a project to the highest available level' do
+      to_remove = create_authorization(project, user, Gitlab::Access::DEVELOPER)
+
+      expect(service).to receive(:update_with_lease).
+        with([to_remove.id], [[user.id, project.id, Gitlab::Access::MASTER]])
+
+      service.execute
+    end
+
+    it 'returns a User' do
+      expect(service.execute).to be_an_instance_of(User)
+    end
+  end
+
+  describe '#update_with_lease', :redis do
+    it 'refreshes the authorizations using a lease' do
+      expect_any_instance_of(Gitlab::ExclusiveLease).to receive(:try_obtain).
+        and_return('foo')
+
+      expect(Gitlab::ExclusiveLease).to receive(:cancel).
+        with(an_instance_of(String), 'foo')
+
+      expect(service).to receive(:update_authorizations).with([1], [])
+
+      service.update_with_lease([1])
+    end
+  end
+
+  describe '#update_authorizations' do
+    it 'does nothing when there are no rows to add and remove' do
+      expect(user).not_to receive(:remove_project_authorizations)
+      expect(ProjectAuthorization).not_to receive(:insert_authorizations)
+      expect(user).not_to receive(:set_authorized_projects_column)
+
+      service.update_authorizations([], [])
+    end
+
+    it 'removes authorizations that should be removed' do
+      authorization = create_authorization(project, user)
+
+      service.update_authorizations([authorization.id])
+
+      expect(user.project_authorizations).to be_empty
+    end
+
+    it 'inserts authorizations that should be added' do
+      service.update_authorizations([], [[user.id, project.id, Gitlab::Access::MASTER]])
+
+      authorizations = user.project_authorizations
+
+      expect(authorizations.length).to eq(1)
+      expect(authorizations[0].user_id).to eq(user.id)
+      expect(authorizations[0].project_id).to eq(project.id)
+      expect(authorizations[0].access_level).to eq(Gitlab::Access::MASTER)
+    end
+
+    it 'populates the authorized projects column' do
+      # make sure we start with a nil value no matter what the default in the
+      # factory may be.
+      user.update(authorized_projects_populated: nil)
+
+      service.update_authorizations([], [[user.id, project.id, Gitlab::Access::MASTER]])
+
+      expect(user.authorized_projects_populated).to eq(true)
+    end
+  end
+
+  describe '#fresh_access_levels_per_project' do
+    let(:hash) { service.fresh_access_levels_per_project }
+
+    it 'returns a Hash' do
+      expect(hash).to be_an_instance_of(Hash)
+    end
+
+    it 'sets the keys to the project IDs' do
+      expect(hash.keys).to eq([project.id])
+    end
+
+    it 'sets the values to the access levels' do
+      expect(hash.values).to eq([Gitlab::Access::MASTER])
+    end
+  end
+
+  describe '#current_authorizations_per_project' do
+    before { create_authorization(project, user) }
+
+    let(:hash) { service.current_authorizations_per_project }
+
+    it 'returns a Hash' do
+      expect(hash).to be_an_instance_of(Hash)
+    end
+
+    it 'sets the keys to the project IDs' do
+      expect(hash.keys).to eq([project.id])
+    end
+
+    it 'sets the values to the project authorization rows' do
+      expect(hash.values).to eq([ProjectAuthorization.first])
+    end
+  end
+
+  describe '#current_authorizations' do
+    context 'without authorizations' do
+      it 'returns an empty list' do
+        expect(service.current_authorizations.empty?).to eq(true)
+      end
+    end
+
+    context 'with an authorization' do
+      before { create_authorization(project, user) }
+
+      let(:row) { service.current_authorizations.take }
+
+      it 'returns the currently authorized projects' do
+        expect(service.current_authorizations.length).to eq(1)
+      end
+
+      it 'includes the row ID for every row' do
+        expect(row.id).to be_a_kind_of(Numeric)
+      end
+
+      it 'includes the project ID for every row' do
+        expect(row.project_id).to eq(project.id)
+      end
+
+      it 'includes the access level for every row' do
+        expect(row.access_level).to eq(Gitlab::Access::MASTER)
+      end
+    end
+  end
+
+  describe '#fresh_authorizations' do
+    it 'returns the new authorized projects' do
+      expect(service.fresh_authorizations.length).to eq(1)
+    end
+
+    it 'returns the highest access level' do
+      project.team.add_guest(user)
+
+      rows = service.fresh_authorizations.to_a
+
+      expect(rows.length).to eq(1)
+      expect(rows.first.access_level).to eq(Gitlab::Access::MASTER)
+    end
+
+    context 'every returned row' do
+      let(:row) { service.fresh_authorizations.take }
+
+      it 'includes the project ID' do
+        expect(row.project_id).to eq(project.id)
+      end
+
+      it 'includes the access level' do
+        expect(row.access_level).to eq(Gitlab::Access::MASTER)
+      end
+    end
+  end
+end
diff --git a/spec/workers/authorized_projects_worker_spec.rb b/spec/workers/authorized_projects_worker_spec.rb
index 95e2458da35..b6591f272f6 100644
--- a/spec/workers/authorized_projects_worker_spec.rb
+++ b/spec/workers/authorized_projects_worker_spec.rb
@@ -7,27 +7,17 @@ describe AuthorizedProjectsWorker do
     it "refreshes user's authorized projects" do
       user = create(:user)
 
-      expect(worker).to receive(:refresh).with(an_instance_of(User))
+      expect_any_instance_of(User).to receive(:refresh_authorized_projects)
 
       worker.perform(user.id)
     end
 
     context "when the user is not found" do
       it "does nothing" do
-        expect(worker).not_to receive(:refresh)
+        expect_any_instance_of(User).not_to receive(:refresh_authorized_projects)
 
         described_class.new.perform(-1)
       end
     end
   end
-
-  describe '#refresh', redis: true do
-    it 'refreshes the authorized projects of the user' do
-      user = create(:user)
-
-      expect(user).to receive(:refresh_authorized_projects)
-
-      worker.refresh(user)
-    end
-  end
 end