Fix N+1 problem for system notes metadata association

Follow-up for https://gitlab.com/gitlab-org/gitlab/-/issues/347407

In previous MR I added a permission verification when user requests
the list of participants. But it increased the number of db queries.

I found that system notes metadata association does not preload
the object and that generates additional queries.

Changelog: fixed

app/models/concerns/issuable.rb

@@ -66,6 +66,11 @@ module Issuable
         # We check first if we're loaded to not load unnecessarily.
         loaded? && to_a.all? { |note| note.association(:project).loaded? }
       end
+
+      def system_note_metadata_loaded?
+        # We check first if we're loaded to not load unnecessarily.
+        loaded? && to_a.all? { |note| note.association(:system_note_metadata).loaded? }
+      end
     end
 
     has_many :note_authors, -> { distinct }, through: :notes, source: :author
@@ -530,6 +535,7 @@ module Issuable
     includes << :author unless notes.authors_loaded?
     includes << :award_emoji unless notes.award_emojis_loaded?
     includes << :project unless notes.projects_loaded?
+    includes << :system_note_metadata unless notes.system_note_metadata_loaded?
 
     if includes.any?
       notes.includes(includes)

spec/graphql/resolvers/users/participants_resolver_spec.rb

@@ -13,6 +13,7 @@ RSpec.describe Resolvers::Users::ParticipantsResolver do
     let_it_be(:note) do
       create(
         :note,
+        :system,
         :confidential,
         project: project,
         noteable: issue,
@@ -20,6 +21,8 @@ RSpec.describe Resolvers::Users::ParticipantsResolver do
       )
     end
 
+    let_it_be(:note_metadata) { create(:system_note_metadata, note: note) }
+
     subject(:resolved_items) { resolve(described_class, args: {}, ctx: { current_user: current_user }, obj: issue)&.items }
 
     before do
@@ -50,17 +53,30 @@ RSpec.describe Resolvers::Users::ParticipantsResolver do
         is_expected.to match_array([issue.author, note.author])
       end
 
-      it 'does not execute N+1 for project relation' do
-        query = -> { resolve(described_class, args: {}, ctx: { current_user: current_user }, obj: issue)&.items }
+      context 'N+1 queries' do
+        let(:query) { -> { resolve(described_class, args: {}, ctx: { current_user: current_user }, obj: issue)&.items } }
+
+        before do
+          # warm-up
+          query.call
+        end
+
+        it 'does not execute N+1 for project relation' do
+          control_count = ActiveRecord::QueryRecorder.new { query.call }
+
+          create(:note, :confidential, project: project, noteable: issue, author: create(:user))
 
-        # warm-up
-        query.call
+          expect { query.call }.not_to exceed_query_limit(control_count)
+        end
 
-        control_count = ActiveRecord::QueryRecorder.new { query.call }
+        it 'does not execute N+1 for system note metadata relation' do
+          control_count = ActiveRecord::QueryRecorder.new { query.call }
 
-        create(:note, :confidential, project: project, noteable: issue, author: create(:user))
+          new_note = create(:note, :system, project: project, noteable: issue, author: create(:user))
+          create(:system_note_metadata, note: new_note)
 
-        expect { query.call }.not_to exceed_query_limit(control_count)
+          expect { query.call }.not_to exceed_query_limit(control_count)
+        end
       end
     end
   end