Clear merge requests counter cache after merge

app/services/merge_requests/post_merge_service.rb

@@ -14,6 +14,7 @@ module MergeRequests
       notification_service.merge_mr(merge_request, current_user)
       execute_hooks(merge_request, 'merge')
       invalidate_cache_counts(merge_request, users: merge_request.assignees)
+      merge_request.update_project_counter_caches
     end
 
     private

changelogs/unreleased/fix-mr-sidebar-counter-after-merge.yml

+---
+title: Fix merge request counter updates after merge
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/post_merge_service_spec.rb

@@ -11,5 +11,16 @@ describe MergeRequests::PostMergeService do
 
   describe '#execute' do
     it_behaves_like 'cache counters invalidator'
+
+    it 'refreshes the number of open merge requests for a valid MR', :use_clean_rails_memory_store_caching do
+      # Cache the counter before the MR changed state.
+      project.open_merge_requests_count
+      merge_request.update!(state: 'merged')
+
+      service = described_class.new(project, user, {})
+
+      expect { service.execute(merge_request) }
+        .to change { project.open_merge_requests_count }.from(1).to(0)
+    end
   end
 end