Sync destruction between requirements and issues

Changelog: added
EE: true

ee/app/models/requirements_management/requirement.rb

@@ -20,7 +20,11 @@ module RequirementsManagement
 
     belongs_to :author, inverse_of: :requirements, class_name: 'User'
     belongs_to :project, inverse_of: :requirements
-    belongs_to :requirement_issue, class_name: 'Issue', foreign_key: :issue_id
+    # deleting an issue would result in deleting requirement record due to cascade delete via foreign key
+    # but to sync the other way around, we require a temporary `dependent: :destroy`
+    # See https://gitlab.com/gitlab-org/gitlab/-/issues/323779 for details.
+    # This will be removed in https://gitlab.com/gitlab-org/gitlab/-/issues/329432
+    belongs_to :requirement_issue, class_name: 'Issue', foreign_key: :issue_id, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
     validates :issue_id, uniqueness: true, allow_nil: true
 

ee/spec/models/requirements_management/requirement_spec.rb

@@ -184,4 +184,21 @@ RSpec.describe RequirementsManagement::Requirement do
       end
     end
   end
+
+  describe 'sync with requirement issues' do
+    let_it_be_with_reload(:requirement) { create(:requirement) }
+    let_it_be_with_reload(:requirement_issue) { create(:requirement_issue, requirement: requirement) }
+
+    context 'when destroying a requirement' do
+      it 'also destroys the associated requirement issue' do
+        expect { requirement.destroy! }.to change { Issue.where(issue_type: 'requirement').count }.by(-1)
+      end
+    end
+
+    context 'when destroying a requirement issue' do
+      it 'also destroys the associated requirement' do
+        expect { requirement_issue.destroy! }.to change { RequirementsManagement::Requirement.count }.by(-1)
+      end
+    end
+  end
 end