Fix N+1 issues in API v3/repos/{full-path}/events

https://gitlab.com/gitlab-org/gitlab/-/issues/321839

config/feature_flags/development/api_v3_repos_events_optimization.yml

+---
+name: api_v3_repos_events_optimization
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54618
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/322059
+milestone: '13.10'
+type: development
+group: group::ecosystem
+default_enabled: false

lib/api/v3/github.rb

@@ -194,13 +194,19 @@ module API
 
         # Self-hosted Jira (tested on 7.11.1) requests this endpoint right
         # after fetching branches.
+        # rubocop: disable CodeReuse/ActiveRecord
         get ':namespace/:project/events' do
           user_project = find_project_with_access(params)
 
           merge_requests = authorized_merge_requests_for_project(user_project)
 
+          if Feature.enabled?(:api_v3_repos_events_optimization, user_project)
+            merge_requests = merge_requests.preload(:author, :assignees, :metrics, source_project: :namespace, target_project: :namespace)
+          end
+
           present paginate(merge_requests), with: ::API::Github::Entities::PullRequestEvent
         end
+        # rubocop: enable CodeReuse/ActiveRecord
 
         params do
           use :project_full_path

spec/requests/api/v3/github_spec.rb

@@ -149,6 +149,8 @@ RSpec.describe API::V3::Github do
     end
 
     describe 'GET events' do
+      include ProjectForksHelper
+
       let(:group) { create(:group) }
       let(:project) { create(:project, :empty_repo, path: 'project.with.dot', group: group) }
       let(:events_path) { "/repos/#{group.path}/#{project.path}/events" }
@@ -174,6 +176,34 @@ RSpec.describe API::V3::Github do
         end
       end
 
+      it 'avoids N+1 queries' do
+        create(:merge_request, source_project: project)
+        source_project = fork_project(project, nil, repository: true)
+
+        control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) { jira_get v3_api(events_path, user) }.count
+
+        create_list(:merge_request, 2, :unique_branches, source_project: source_project, target_project: project)
+
+        expect { jira_get v3_api(events_path, user) }.not_to exceed_all_query_limit(control_count)
+      end
+
+      context 'with `api_v3_repos_events_optimization` feature flag disabled' do
+        before do
+          stub_feature_flags(api_v3_repos_events_optimization: false)
+        end
+
+        it 'falls back to less optimal query performance' do
+          create(:merge_request, source_project: project)
+          source_project = fork_project(project, nil, repository: true)
+
+          control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) { jira_get v3_api(events_path, user) }.count
+
+          create_list(:merge_request, 2, :unique_branches, source_project: source_project, target_project: project)
+
+          expect { jira_get v3_api(events_path, user) }.to exceed_all_query_limit(control_count)
+        end
+      end
+
       context 'if there are more merge requests' do
         let!(:merge_request) { create(:merge_request, id: 10000, source_project: project, target_project: project, author: user) }
         let!(:merge_request2) { create(:merge_request, id: 10001, source_project: project, source_branch: generate(:branch), target_project: project, author: user) }