Fix N+1 for notification recipients in subscribers

app/models/concerns/subscribable.rb

@@ -31,9 +31,11 @@ module Subscribable
   end
 
   def subscribers(project)
-    subscriptions_available(project)
+    relation = subscriptions_available(project)
                  .where(subscribed: true)
-      .map(&:user)
+                 .select(:user_id)
+
+    User.where(id: relation)
   end
 
   def toggle_subscription(user, project = nil)

spec/services/notification_recipient_service_spec.rb

@@ -10,6 +10,7 @@ describe NotificationRecipientService do
     let(:issue) { create(:issue, project: project, assignees: [assignee]) }
     let(:note) { create(:note_on_issue, noteable: issue, project_id: issue.project_id) }
 
+    context 'when there are multiple watchers' do
       def create_watcher
         watcher = create(:user)
         create(:notification_setting, source: project, user: watcher, level: :watch)
@@ -33,4 +34,26 @@ describe NotificationRecipientService do
         expect { service.build_new_note_recipients(note) }.not_to exceed_query_limit(control_count)
       end
     end
+
+    context 'when there are multiple subscribers' do
+      def create_subscriber
+        subscriber = create(:user)
+        issue.subscriptions.create(user: subscriber, project: project, subscribed: true)
+      end
+
+      it 'avoids N+1 queries' do
+        create_subscriber
+
+        service.build_new_note_recipients(note)
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          service.build_new_note_recipients(note)
+        end
+
+        create_subscriber
+
+        expect { service.build_new_note_recipients(note) }.not_to exceed_query_limit(control_count)
+      end
+    end
+  end
 end