Fix epic references when adding as related

Allow to relate epics with same iid
from different groups.

ee/app/controllers/groups/autocomplete_sources_controller.rb

@@ -29,7 +29,10 @@ class Groups::AutocompleteSourcesController < Groups::ApplicationController
   end
 
   def epics
-    render json: @autocomplete_service.epics(confidential_only: params[:confidential_only])
+    render json: issuable_serializer.represent(
+      @autocomplete_service.epics(confidential_only: params[:confidential_only]),
+      parent_group: @group
+    )
   end
 
   def vulnerabilities

ee/app/services/groups/autocomplete_service.rb

@@ -34,7 +34,7 @@ module Groups
       # See https://gitlab.com/gitlab-org/gitlab/issues/6837
       EpicsFinder.new(current_user, finder_params)
         .execute
-        .select(:iid, :title)
+        .select(:iid, :title, :group_id)
     end
 
     def vulnerabilities

ee/spec/controllers/groups/autocomplete_sources_controller_spec.rb

@@ -28,7 +28,7 @@ RSpec.describe Groups::AutocompleteSourcesController do
 
       expect(json_response).to be_an(Array)
       expect(json_response.first).to include(
-        'iid' => epic.iid, 'title' => epic.title
+        'iid' => epic.iid, 'title' => epic.title, 'reference' => epic.to_reference(epic.group)
       )
     end
   end

ee/spec/services/groups/autocomplete_service_spec.rb

@@ -92,6 +92,8 @@ RSpec.describe Groups::AutocompleteService do
   end
 
   describe '#epics' do
+    let(:expected_attributes) { %i(iid title group_id) }
+
     before do
       stub_licensed_features(epics: true)
     end
@@ -105,16 +107,18 @@ RSpec.describe Groups::AutocompleteService do
     end
 
     it 'returns epics from group' do
-      expect(subject.epics.map(&:iid)).to contain_exactly(epic.iid)
+      result = subject.epics.map { |epic| epic.slice(expected_attributes) }
+
+      expect(result).to contain_exactly(epic.slice(expected_attributes))
     end
 
     it 'returns only confidential epics if confidential_only is true' do
       confidential_epic = create(:epic, :confidential, group: group)
 
-      epics = subject.epics(confidential_only: true)
+      result = subject.epics(confidential_only: true)
+                 .map { |epic| epic.slice(expected_attributes) }
 
-      expect(epics.map(&:iid)).to contain_exactly(confidential_epic.iid)
-      expect(epics.map(&:title)).to contain_exactly(confidential_epic.title)
+      expect(result).to contain_exactly(confidential_epic.slice(expected_attributes))
     end
   end