Add a concern to build hierarchies of groups

app/models/concerns/group_hierarchy.rb

@@ -0,0 +1,27 @@
+module GroupHierarchy
+  def hierarchy(hierarchy_base = nil)
+    @hierarchy ||= tree_for_child(self, self, hierarchy_base)
+  end
+
+  def parent
+    if self.is_a?(Project)
+      namespace
+    else
+      super
+    end
+  end
+
+  def tree_for_child(child, tree, hierarchy_base)
+    if child.parent.nil? && hierarchy_base.present?
+      raise ArgumentError.new('specified base is not part of the tree')
+    end
+
+    if child.parent != hierarchy_base
+      tree_for_child(child.parent,
+                     { child.parent => tree },
+                     hierarchy_base)
+    else
+      tree
+    end
+  end
+end

app/models/group.rb

@@ -6,6 +6,7 @@ class Group < Namespace
   include Avatarable
   include Referable
   include SelectForProjectAuthorization
+  include GroupHierarchy
 
   has_many :group_members, -> { where(requested_at: nil) }, dependent: :destroy, as: :source # rubocop:disable Cop/ActiveRecordDependent
   alias_method :members, :group_members

app/models/project.rb

@@ -17,6 +17,7 @@ class Project < ActiveRecord::Base
   include ProjectFeaturesCompatibility
   include SelectForProjectAuthorization
   include Routable
+  include GroupHierarchy
 
   extend Gitlab::ConfigHelper
   extend Gitlab::CurrentSettings

spec/models/concerns/group_hierarchy_spec.rb

@@ -0,0 +1,61 @@
+require 'spec_helper'
+
+describe GroupHierarchy, :nested_groups do
+  let(:parent) { create(:group) }
+  let(:subgroup) { create(:group, parent: parent) }
+  let(:subsub_group) { create(:group, parent: subgroup) }
+
+  context 'for a group' do
+    describe '#hierarchy' do
+      it 'builds a hierarchy for a group' do
+        expected_hierarchy = { parent => { subgroup => subsub_group } }
+
+        expect(subsub_group.hierarchy).to eq(expected_hierarchy)
+      end
+
+      it 'builds a hierarchy upto a specified parent' do
+        expected_hierarchy = { subgroup => subsub_group }
+
+        expect(subsub_group.hierarchy(parent)).to eq(expected_hierarchy)
+      end
+
+      it 'raises an error if specifying a base that is not part of the tree' do
+        expect { subsub_group.hierarchy(double) }.to raise_error('specified base is not part of the tree')
+      end
+    end
+
+    describe '#parent' do
+      it 'returns the correct parent' do
+        expect(subsub_group.parent).to eq(subgroup)
+      end
+    end
+  end
+
+  context 'for a project' do
+    let(:project) { create(:project, namespace: subsub_group) }
+
+    describe '#hierarchy' do
+      it 'builds a hierarchy for a group' do
+        expected_hierarchy = { parent => { subgroup => { subsub_group => project } } }
+
+        expect(project.hierarchy).to eq(expected_hierarchy)
+      end
+
+      it 'builds a hierarchy upto a specified parent' do
+        expected_hierarchy = { subsub_group => project }
+
+        expect(project.hierarchy(subgroup)).to eq(expected_hierarchy)
+      end
+
+      it 'raises an error if specifying a base that is not part of the tree' do
+        expect { project.hierarchy(double) }.to raise_error('specified base is not part of the tree')
+      end
+    end
+
+    describe '#parent' do
+      it 'returns the correct parent' do
+        expect(project.parent).to eq(subsub_group)
+      end
+    end
+  end
+end