Fix N+1 spec to test correctly

Move it to request spec since we are transitioning controller
specs to request specs. Also, query counter should be used in
request specs as it is possible to return false positives when
used in controller specs.

ee/app/controllers/concerns/ee/issuable_collections.rb

@@ -9,7 +9,7 @@ module EE
     def preload_for_collection
       @preload_for_collection ||= case collection_type
                                   when 'MergeRequest'
-                                    super.push(:approval_rules)
+                                    super + [approval_rules: [:users, :group_users], approval_project_rules: [:users, :group_users]]
                                   when 'Issue'
                                     super.push(*issue_preloads)
                                   else

ee/changelogs/unreleased/276944-reduce-lcp-to-2-5s-for-sourcecode_mr_list.yml

+---
+title: Resolve N+1 issue loading approval rules on the MR list
+merge_request: 52364
+author:
+type: performance

ee/spec/controllers/projects/merge_requests_controller_spec.rb

@@ -60,39 +60,6 @@ RSpec.describe Projects::MergeRequestsController do
     sign_in(viewer)
   end
 
-  describe 'GET index' do
-    def get_merge_requests
-      get :index,
-        params: {
-          namespace_id: project.namespace.to_param,
-          project_id: project,
-          state: 'opened'
-        }
-    end
-
-    context 'when filtering by opened state' do
-      context 'with opened merge requests' do
-        render_views
-        it 'avoids N+1' do
-          other_user = create(:user)
-          create(:merge_request, :unique_branches, target_project: project, source_project: project)
-          create_list(:approval_merge_request_rule, 5, merge_request: merge_request, users: [user, other_user], approvals_required: 2)
-
-          control_count = ActiveRecord::QueryRecorder.new { get_merge_requests }.count
-
-          create_list(:approval, 10)
-          create_list(:merge_request, 20, :unique_branches, target_project: project, source_project: project).each do |mr|
-            create(:approval_merge_request_rule, merge_request: merge_request, users: [user, other_user], approvals_required: 2)
-          end
-
-          expect do
-            get_merge_requests
-          end.not_to exceed_query_limit(control_count)
-        end
-      end
-    end
-  end
-
   describe 'PUT update' do
     let_it_be_with_reload(:merge_request) do
       create(:merge_request_with_diffs, source_project: project, author: author)

ee/spec/requests/projects/merge_requests_controller_spec.rb

@@ -43,4 +43,27 @@ RSpec.describe Projects::MergeRequestsController do
       end
     end
   end
+
+  describe 'GET #index' do
+    def get_index
+      get project_merge_requests_path(project, state: 'opened')
+    end
+
+    it 'avoids N+1' do
+      other_user = create(:user)
+      create(:merge_request, :unique_branches, target_project: project, source_project: project)
+      create_list(:approval_project_rule, 5, project: project, users: [user, other_user], approvals_required: 2)
+      create_list(:approval_merge_request_rule, 5, merge_request: merge_request, users: [user, other_user], approvals_required: 2)
+
+      control_count = ActiveRecord::QueryRecorder.new { get_index }.count
+
+      create_list(:approval, 10)
+      create(:approval_project_rule, project: project, users: [user, other_user], approvals_required: 2)
+      create_list(:merge_request, 20, :unique_branches, target_project: project, source_project: project).each do |mr|
+        create(:approval_merge_request_rule, merge_request: mr, users: [user, other_user], approvals_required: 2)
+      end
+
+      expect { get_index }.not_to exceed_query_limit(control_count)
+    end
+  end
 end