Inherit parent epic during an issue promotion

- changess to IssuePromoteService
- update doc

doc/user/group/epics/manage_epics.md

@@ -271,6 +271,7 @@ The following issue metadata will be copied to the epic:
 - Upvotes/downvotes.
 - Participants.
 - Group labels that the issue already has.
+- Parent epic. **(ULTIMATE)**
 
 ## Manage multi-level child epics **(ULTIMATE)**
 

ee/app/services/epics/issue_promote_service.rb

@@ -52,10 +52,15 @@ module Epics
 
     def params
       {
-        title: original_entity.title
+        title: original_entity.title,
+        parent: issue_epic
       }
     end
 
+    def issue_epic
+      original_entity.epic_issue&.epic
+    end
+
     def add_note_from
       SystemNoteService.issue_promoted(new_entity, original_entity, current_user, direction: :from)
     end

ee/changelogs/unreleased/199878-issue-promotion.yml

+---
+title: Inherit parent epic during an issue promotion
+merge_request: 37109
+author:
+type: changed

ee/spec/features/epics/issue_promotion_spec.rb

@@ -8,6 +8,8 @@ RSpec.describe 'Issue promotion', :js do
   let(:group) { create(:group) }
   let(:project) { create(:project, :public, group: group) }
   let(:issue) { create(:issue, project: project) }
+  let(:parent_epic) { create(:epic, group: group) }
+  let!(:epic_issue) { create(:epic_issue, issue: issue, epic: parent_epic) }
   let(:user) { create(:user) }
 
   before do
@@ -21,7 +23,7 @@ RSpec.describe 'Issue promotion', :js do
       expect(page).not_to have_content 'Promoted issue to an epic.'
 
       expect(issue.reload).to be_open
-      expect(Epic.count).to be_zero
+      expect(Epic.count).to eq(1)
     end
   end
 
@@ -39,7 +41,7 @@ RSpec.describe 'Issue promotion', :js do
         expect(page).not_to have_content 'Promoted issue to an epic.'
 
         expect(issue.reload).to be_open
-        expect(Epic.count).to be_zero
+        expect(Epic.count).to eq(1)
       end
     end
 
@@ -70,6 +72,7 @@ RSpec.describe 'Issue promotion', :js do
         expect(epic.title).to eq(issue.title)
         expect(epic.description).to eq(issue.description)
         expect(epic.author).to eq(user)
+        expect(epic.parent).to eq(parent_epic)
       end
 
       # Spec for https://gitlab.com/gitlab-org/gitlab/-/issues/215549

ee/spec/services/epics/issue_promote_service_spec.rb

@@ -69,6 +69,7 @@ RSpec.describe Epics::IssuePromoteService do
             expect(epic.description).to eq(issue.description)
             expect(epic.author).to eq(user)
             expect(epic.group).to eq(group)
+            expect(epic.parent).to be_nil
           end
 
           it 'copies group labels assigned to the issue' do
@@ -104,6 +105,21 @@ RSpec.describe Epics::IssuePromoteService do
           end
         end
 
+        context 'when an issue belongs to an epic' do
+          let(:parent_epic) { create(:epic, group: group) }
+          let!(:epic_issue) { create(:epic_issue, epic: parent_epic, issue: issue) }
+
+          it 'creates a new epic with correct attributes' do
+            subject.execute(issue)
+
+            expect(epic.title).to eq(issue.title)
+            expect(epic.description).to eq(issue.description)
+            expect(epic.author).to eq(user)
+            expect(epic.group).to eq(group)
+            expect(epic.parent).to eq(parent_epic)
+          end
+        end
+
         context 'when issue was already promoted' do
           it 'raises error' do
             epic = create(:epic, group: group)