Fix N+1 for notification recipients on private projects

If we don't call #to_a, we're relying on the members already being loaded from
elsewhere. Otherwise we'll do a separate query for each user:

    [1] pry(main)> Project.first.team.members.include?(User.first)
      Project Load (0.7ms)  SELECT  "projects".* FROM "projects"  ORDER BY "projects"."id" ASC LIMIT 1
      ↳ (pry):3
      User Load (1.8ms)  SELECT  "users".* FROM "users"  ORDER BY "users"."id" ASC LIMIT 1
      ↳ (pry):3
      User Exists (0.6ms)  SELECT  1 AS one FROM "users" INNER JOIN "project_authorizations" ON "users"."id" = "project_authorizations"."user_id" WHERE "project_authorizations"."project_id" = $1 AND "users"."id" = $2 LIMIT 1  [["project_id", 1], ["id", 1]]
      ↳ (pry):3
    => true
    [2] pry(main)> Project.first.team.members.to_a.include?(User.first)
      Project Load (12.8ms)  SELECT  "projects".* FROM "projects"  ORDER BY "projects"."id" ASC LIMIT 1
      ↳ (pry):1
      User Load (9.6ms)  SELECT "users".* FROM "users" INNER JOIN "project_authorizations" ON "users"."id" = "project_authorizations"."user_id" WHERE "project_authorizations"."project_id" = $1  [["project_id", 1]]
      ↳ (pry):1
      User Load (0.6ms)  SELECT  "users".* FROM "users"  ORDER BY "users"."id" ASC LIMIT 1
      ↳ (pry):1
    => true

app/policies/project_policy.rb

@@ -390,7 +390,11 @@ class ProjectPolicy < BasePolicy
     greedy_load_subject ||= !@user.persisted?
 
     if greedy_load_subject
-      project.team.members.include?(user)
+      # We want to load all the members with one query. Calling #include? on
+      # project.team.members will perform a separate query for each user, unless
+      # project.team.members was loaded before somewhere else. Calling #to_a
+      # ensures it's always loaded before checking for membership.
+      project.team.members.to_a.include?(user)
     else
       # otherwise we just make a specific query for
       # this particular user.

spec/services/notification_recipient_service_spec.rb

@@ -10,18 +10,9 @@ 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)
-
-        other_projects.each do |other_project|
-          create(:notification_setting, source: other_project, user: watcher, level: :watch)
-        end
-      end
-
+    shared_examples 'no N+1 queries' do
       it 'avoids N+1 queries', :request_store do
-        create_watcher
+        create_user
 
         service.build_new_note_recipients(note)
 
@@ -29,30 +20,39 @@ describe NotificationRecipientService do
           service.build_new_note_recipients(note)
         end
 
-        create_watcher
+        create_user
 
         expect { service.build_new_note_recipients(note) }.not_to exceed_query_limit(control_count)
       end
     end
 
+    context 'when there are multiple watchers' do
+      def create_user
+        watcher = create(:user)
+        create(:notification_setting, source: project, user: watcher, level: :watch)
+
+        other_projects.each do |other_project|
+          create(:notification_setting, source: other_project, user: watcher, level: :watch)
+        end
+      end
+
+      include_examples 'no N+1 queries'
+    end
+
     context 'when there are multiple subscribers' do
-      def create_subscriber
+      def create_user
         subscriber = create(:user)
         issue.subscriptions.create(user: subscriber, project: project, subscribed: true)
       end
 
-      it 'avoids N+1 queries' do
-        create_subscriber
+      include_examples 'no N+1 queries'
 
-        service.build_new_note_recipients(note)
-
-        control_count = ActiveRecord::QueryRecorder.new do
-          service.build_new_note_recipients(note)
+      context 'when the project is private' do
+        before do
+          project.update!(visibility_level: Gitlab::VisibilityLevel::PRIVATE)
         end
 
-        create_subscriber
-
-        expect { service.build_new_note_recipients(note) }.not_to exceed_query_limit(control_count)
+        include_examples 'no N+1 queries'
       end
     end
   end