Update previous epic milestone dates when issue's epic switches

Both the old and new epic should update their milestone dates
when an issue switches epic.

ee/app/services/epic_issues/create_service.rb

 module EpicIssues
   class CreateService < IssuableLinks::CreateService
-    def execute
-      result = super
-      issuable.update_start_and_due_dates
-      result
-    end
-
     private
 
     # rubocop: disable CodeReuse/ActiveRecord
     def relate_issues(referenced_issue)
       link = EpicIssue.find_or_initialize_by(issue: referenced_issue)
 
-      params = if link.persisted?
-                 { issue_moved: true, original_epic: link.epic }
-               else
-                 {}
-               end
+      affected_epics = [issuable]
+
+      if link.persisted?
+        affected_epics << link.epic
+        params = { issue_moved: true, original_epic: link.epic }
+      else
+        params = {}
+      end
 
       link.epic = issuable
       link.move_to_start
       link.save!
 
+      affected_epics.each(&:update_start_and_due_dates)
+
       yield params
     end
     # rubocop: enable CodeReuse/ActiveRecord

ee/changelogs/unreleased/7869-update-previous-epic-when-issue-moves-to-new-epic.yml

+---
+title: Fix epic milestone dates incorrect after issue is linked to another epic
+merge_request: 7809
+author:
+type: fixed

ee/spec/services/epic_issues/create_service_spec.rb

@@ -22,7 +22,8 @@ describe EpicIssues::CreateService do
     shared_examples 'returns success' do
       let(:created_link) { EpicIssue.find_by!(issue_id: issue.id) }
 
-      it 'creates a new relationship' do
+      it 'creates a new relationship and updates epic' do
+        expect(epic).to receive(:update_start_and_due_dates)
         expect { subject }.to change(EpicIssue, :count).from(1).to(2)
 
         expect(created_link).to have_attributes(epic: epic)
@@ -141,7 +142,7 @@ describe EpicIssues::CreateService do
               # and we insert 5 issues instead of 1 which we do for control count
               expect { described_class.new(epic, user, params).execute }
                 .not_to exceed_query_limit(control_count)
-                .with_threshold(24)
+                .with_threshold(28)
             end
           end
 
@@ -228,6 +229,18 @@ describe EpicIssues::CreateService do
           expect { subject }.to change { Note.count }.from(0).to(3)
         end
 
+        it 'updates both old and new epic milestone dates' do
+          allow(EpicIssue).to receive(:find_or_initialize_by).with(issue: issue).and_wrap_original { |m, *args|
+            existing_epic_issue = m.call(*args)
+            expect(existing_epic_issue.epic).to receive(:update_start_and_due_dates)
+            existing_epic_issue
+          }
+
+          expect(another_epic).to receive(:update_start_and_due_dates)
+
+          subject
+        end
+
         it 'creates a note correctly for the original epic' do
           subject
 
@@ -268,14 +281,6 @@ describe EpicIssues::CreateService do
 
         include_examples 'returns an error'
       end
-
-      context 'refresh epic dates' do
-        it 'calls epic#update_start_and_due_dates' do
-          expect(epic).to receive(:update_start_and_due_dates)
-
-          assign_issue([valid_reference])
-        end
-      end
     end
   end
 end