Relate issues when being marked as duplicate

app/services/issues/duplicate_service.rb

@@ -25,3 +25,5 @@ module Issues
     end
   end
 end
+
+Issues::DuplicateService.prepend_if_ee('EE::Issues::DuplicateService')

changelogs/unreleased/3963-auto-related-duplicated-issues.yml

+---
+title: Relate issues when they are marked as duplicated
+merge_request: 20161
+author: minghuan lei
+type: added

doc/user/project/quick_actions.md

@@ -60,7 +60,7 @@ The following quick actions are applicable to descriptions, discussions and thre
 | `/remove_epic`                        | ✓     |               |      | Remove from epic **(ULTIMATE)** |
 | `/promote`                            | ✓     |               |      | Promote issue to epic **(ULTIMATE)** |
 | `/confidential`                       | ✓     |               |      | Make confidential |
-| `/duplicate <#issue>`                 | ✓     |               |      | Mark this issue as a duplicate of another issue |
+| `/duplicate <#issue>`                 | ✓     |               |      | Mark this issue as a duplicate of another issue and relate them for **(STARTER)** |
 | `/create_merge_request <branch name>` | ✓     |               |      | Create a new merge request starting from the current issue |
 | `/relate #issue1 #issue2`             | ✓     |               |      | Mark issues as related **(STARTER)** |
 | `/move <path/to/project>`             | ✓     |               |      | Move this issue to another project |

ee/app/services/ee/issues/duplicate_service.rb

+# frozen_string_literal: true
+
+module EE
+  module Issues
+    module DuplicateService
+      extend ::Gitlab::Utils::Override
+
+      override :execute
+      def execute(duplicate_issue, canonical_issue)
+        super
+
+        relate_two_issues(duplicate_issue, canonical_issue)
+      end
+
+      private
+
+      def relate_two_issues(duplicate_issue, canonical_issue)
+        params = { target_issuable: canonical_issue }
+        IssueLinks::CreateService.new(duplicate_issue, current_user, params).execute
+      end
+    end
+  end
+end

ee/spec/services/issues/duplicate_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Issues::DuplicateService do
+  let(:user) { create(:user) }
+  let(:canonical_project) { create(:project) }
+  let(:duplicate_project) { create(:project) }
+
+  let(:canonical_issue) { create(:issue, project: canonical_project) }
+  let(:duplicate_issue) { create(:issue, project: duplicate_project) }
+
+  subject { described_class.new(duplicate_project, user, {}) }
+
+  describe '#execute' do
+    it 'relates the duplicate issues' do
+      canonical_project.add_reporter(user)
+      duplicate_project.add_reporter(user)
+
+      subject.execute(duplicate_issue, canonical_issue)
+
+      issue_link = IssueLink.last
+      expect(issue_link.source).to eq(duplicate_issue)
+      expect(issue_link.target).to eq(canonical_issue)
+    end
+  end
+end

spec/services/issues/update_service_spec.rb

@@ -689,8 +689,9 @@ describe Issues::UpdateService, :mailer do
 
       context 'valid canonical_issue_id' do
         it 'calls the duplicate service with both issues' do
-          expect_any_instance_of(Issues::DuplicateService)
-            .to receive(:execute).with(issue, canonical_issue)
+          expect_next_instance_of(Issues::DuplicateService) do |service|
+            expect(service).to receive(:execute).with(issue, canonical_issue)
+          end
 
           update_issue(canonical_issue_id: canonical_issue.id)
         end

spec/support/shared_examples/quick_actions/issue/duplicate_quick_action_shared_examples.rb

-# frozen_string_literal: true
-
-shared_examples 'duplicate quick action' do
-  context 'mark issue as duplicate' do
-    let(:original_issue) { create(:issue, project: project) }
-
-    context 'when the current user can update issues' do
-      it 'does not create a note, and marks the issue as a duplicate' do
-        add_note("/duplicate ##{original_issue.to_reference}")
-
-        expect(page).not_to have_content "/duplicate #{original_issue.to_reference}"
-        expect(page).to have_content "marked this issue as a duplicate of #{original_issue.to_reference}"
-
-        expect(issue.reload).to be_closed
-      end
-    end
-
-    context 'when the current user cannot update the issue' do
-      let(:guest) { create(:user) }
-      before do
-        project.add_guest(guest)
-        gitlab_sign_out
-        sign_in(guest)
-        visit project_issue_path(project, issue)
-      end
-
-      it 'does not create a note, and does not mark the issue as a duplicate' do
-        add_note("/duplicate ##{original_issue.to_reference}")
-
-        expect(page).not_to have_content "marked this issue as a duplicate of #{original_issue.to_reference}"
-
-        expect(issue.reload).to be_open
-      end
-    end
-  end
-end