Fix AttributesRewriter for ResourceMilestoneEvents without milestone

Update specs as well

app/models/resource_milestone_event.rb

@@ -25,4 +25,8 @@ class ResourceMilestoneEvent < ResourceEvent
   def self.issuable_attrs
     %i(issue merge_request).freeze
   end
+
+  def milestone_title
+    milestone&.title
+  end
 end

app/services/issuable/clone/attributes_rewriter.rb

@@ -67,21 +67,29 @@ module Issuable
       end
 
       def copy_resource_milestone_events
-        entity_key = new_entity.class.name.underscore.foreign_key
+        return unless milestone_events_supported?
 
         copy_events(ResourceMilestoneEvent.table_name, original_entity.resource_milestone_events) do |event|
-          matching_destination_milestone = matching_milestone(event.milestone.title)
+          if event.remove?
+            event_attributes_with_milestone(event, nil)
+          else
+            matching_destination_milestone = matching_milestone(event.milestone_title)
+
+            event_attributes_with_milestone(event, matching_destination_milestone) if matching_destination_milestone.present?
+          end
+        end
+      end
+
+      def event_attributes_with_milestone(event, milestone)
+        entity_key = new_entity.class.name.underscore.foreign_key
 
-          if matching_destination_milestone.present?
         event.attributes
           .except('id')
           .merge(entity_key => new_entity.id,
-                     'milestone_id' => matching_destination_milestone.id,
+                 'milestone_id' => milestone&.id,
                  'action' => ResourceMilestoneEvent.actions[event.action],
                  'state' => ResourceMilestoneEvent.states[event.state])
       end
-        end
-      end
 
       def copy_events(table_name, events_to_copy)
         events_to_copy.find_in_batches do |batch|
@@ -96,6 +104,11 @@ module Issuable
       def entity_key
         new_entity.class.name.parameterize('_').foreign_key
       end
+
+      def milestone_events_supported?
+        original_entity.respond_to?(:resource_milestone_events) &&
+          new_entity.respond_to?(:resource_milestone_events)
+      end
     end
   end
 end

ee/spec/features/epics/issue_promotion_spec.rb

@@ -71,6 +71,25 @@ describe 'Issue promotion', :js do
         expect(epic.description).to eq(issue.description)
         expect(epic.author).to eq(user)
       end
+
+      # Spec for https://gitlab.com/gitlab-org/gitlab/-/issues/215549
+      context 'if there is a remove resource milestone event' do
+        let!(:resource_milestone_event) { create(:resource_milestone_event, issue: issue, action: 'remove', milestone_id: nil) }
+
+        it 'promotes the issue' do
+          add_note('/promote')
+
+          wait_for_requests
+
+          epic = Epic.last
+
+          expect(page).to have_content 'Promoted issue to an epic.'
+          expect(issue.reload).to be_closed
+          expect(epic.title).to eq(issue.title)
+          expect(epic.description).to eq(issue.description)
+          expect(epic.author).to eq(user)
+        end
+      end
     end
 
     context 'when issue is confidential' do

spec/models/resource_milestone_event_spec.rb

@@ -78,4 +78,21 @@ describe ResourceMilestoneEvent, type: :model do
       let(:query_method) { :remove? }
     end
   end
+
+  describe '#milestone_title' do
+    let(:milestone) { create(:milestone, title: 'v2.3') }
+    let(:event) { create(:resource_milestone_event, milestone: milestone) }
+
+    it 'returns the expected title' do
+      expect(event.milestone_title).to eq('v2.3')
+    end
+
+    context 'when milestone is nil' do
+      let(:event) { create(:resource_milestone_event, milestone: nil) }
+
+      it 'returns nil' do
+        expect(event.milestone_title).to be_nil
+      end
+    end
+  end
 end

spec/services/issuable/clone/attributes_rewriter_spec.rb

@@ -89,7 +89,7 @@ describe Issuable::Clone::AttributesRewriter do
 
         create_event(milestone1_project1)
         create_event(milestone2_project1)
-        create_event(milestone1_project1, 'remove')
+        create_event(nil, 'remove')
         create_event(milestone3_project1)
       end
 
@@ -101,7 +101,7 @@ describe Issuable::Clone::AttributesRewriter do
 
         expect_milestone_event(new_issue_milestone_events.first, milestone: milestone1_project2, action: 'add', state: 'opened')
         expect_milestone_event(new_issue_milestone_events.second, milestone: milestone2_project2, action: 'add', state: 'opened')
-        expect_milestone_event(new_issue_milestone_events.third, milestone: milestone1_project2, action: 'remove', state: 'opened')
+        expect_milestone_event(new_issue_milestone_events.third, milestone: nil, action: 'remove', state: 'opened')
       end
 
       def create_event(milestone, action = 'add')
@@ -109,7 +109,7 @@ describe Issuable::Clone::AttributesRewriter do
       end
 
       def expect_milestone_event(event, expected_attrs)
-        expect(event.milestone_id).to eq(expected_attrs[:milestone].id)
+        expect(event.milestone_id).to eq(expected_attrs[:milestone]&.id)
         expect(event.action).to eq(expected_attrs[:action])
         expect(event.state).to eq(expected_attrs[:state])
       end