Remove N+1 for API commits/:sha/merge_requests

When a commit belongs to multiple MRs, N+1 is performed

changelogs/unreleased/id-n-1-for-merge-request-commits.yml

+---
+title: Remove N+1 for API commits/:sha/merge_requests
+merge_request: 57290
+author:
+type: performance

lib/api/commits.rb

@@ -372,7 +372,7 @@ module API
           current_user,
           project_id: user_project.id,
           commit_sha: commit.sha
-        ).execute
+        ).execute.with_api_entity_associations
 
         present paginate(commit_merge_requests), with: Entities::MergeRequestBasic
       end

spec/requests/api/commits_spec.rb

@@ -1898,8 +1898,12 @@ RSpec.describe API::Commits do
     let(:merged_mr) { create(:merge_request, source_project: project, source_branch: 'master', target_branch: 'feature') }
     let(:commit) { merged_mr.merge_request_diff.commits.last }
 
-    it 'returns the correct merge request' do
+    def perform_request(user)
       get api("/projects/#{project.id}/repository/commits/#{commit.id}/merge_requests", user)
+    end
+
+    it 'returns the correct merge request' do
+      perform_request(user)
 
       expect(response).to have_gitlab_http_status(:ok)
       expect(response).to include_limited_pagination_headers
@@ -1910,7 +1914,7 @@ RSpec.describe API::Commits do
     it 'returns 403 for an unauthorized user' do
       project.add_guest(user)
 
-      get api("/projects/#{project.id}/repository/commits/#{commit.id}/merge_requests", user)
+      perform_request(user)
 
       expect(response).to have_gitlab_http_status(:forbidden)
     end
@@ -1926,11 +1930,21 @@ RSpec.describe API::Commits do
       let(:non_member) { create(:user) }
 
       it 'responds 403 when only members are allowed to read merge requests' do
-        get api("/projects/#{project.id}/repository/commits/#{commit.id}/merge_requests", non_member)
+        perform_request(non_member)
 
         expect(response).to have_gitlab_http_status(:forbidden)
       end
     end
+
+    it 'returns multiple merge requests without N + 1' do
+      perform_request(user)
+
+      control_count = ActiveRecord::QueryRecorder.new { perform_request(user) }.count
+
+      create(:merge_request, :closed, source_project: project, source_branch: 'master', target_branch: 'feature')
+
+      expect { perform_request(user) }.not_to exceed_query_limit(control_count)
+    end
   end
 
   describe 'GET /projects/:id/repository/commits/:sha/signature' do