Eliminate N+1 queries in loading discussions.json endpointsh-optimize-discussion-json

In #37955,we see that the profile had a number of N+1 queries from repeated
access to `cross_reference_not_visible_for?`. This was optimized in previous
versions of GitLab by rendering all notes at once, counting the number of
visible references, and then using that number to check whether a system note
should be fully redacted.

There was also another N+1 query calling `ProjectTeam#member?`, which did not
take advantage of an optimization in prepare_notes_for_rendering that would
preload the maximum access level per project.

Closes #37955

1 files changed, 32 insertions, 0 deletions

diff --git a/spec/controllers/projects/issues_controller_spec.rb b/spec/controllers/projects/issues_controller_spec.rb
index 5d9403c23ac..b4a22a46b51 100644
--- a/spec/controllers/projects/issues_controller_spec.rb
+++ b/spec/controllers/projects/issues_controller_spec.rb
@@ -900,5 +900,37 @@ describe Projects::IssuesController do
 
       expect(JSON.parse(response.body).first.keys).to match_array(%w[id reply_id expanded notes individual_note])
     end
+
+    context 'with cross-reference system note', :request_store do
+      let(:new_issue) { create(:issue) }
+      let(:cross_reference) { "mentioned in #{new_issue.to_reference(issue.project)}" }
+
+      before do
+        create(:discussion_note_on_issue, :system, noteable: issue, project: issue.project, note: cross_reference)
+      end
+
+      it 'filters notes that the user should not see' do
+        get :discussions, namespace_id: project.namespace, project_id: project, id: issue.iid
+
+        expect(JSON.parse(response.body).count).to eq(1)
+      end
+
+      it 'does not result in N+1 queries' do
+        # Instantiate the controller variables to ensure QueryRecorder has an accurate base count
+        get :discussions, namespace_id: project.namespace, project_id: project, id: issue.iid
+
+        RequestStore.clear!
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          get :discussions, namespace_id: project.namespace, project_id: project, id: issue.iid
+        end.count
+
+        RequestStore.clear!
+
+        create_list(:discussion_note_on_issue, 2, :system, noteable: issue, project: issue.project, note: cross_reference)
+
+        expect { get :discussions, namespace_id: project.namespace, project_id: project, id: issue.iid }.not_to exceed_query_limit(control_count)
+      end
+    end
   end
 end