Fix some N+1s when calculating notification recipients

First N+1: we may have loaded a user's notification settings already, but not
have loaded their sources. Because we're iterating through, we'd potentially
load sources that are completely unrelated, just because they belong to this
user.

Second N+1: we do a separate query for each user who could be subscribed to or
unsubcribed from the target. It's actually more efficient in this case to get
all subscriptions at once, as we will need to check most of them.

We can fix both by the slightly unpleasant means of checking IDs manually,
rather than object equality.

app/models/notification_recipient.rb

@@ -64,7 +64,7 @@ class NotificationRecipient
     return false unless @target
     return false unless @target.respond_to?(:subscriptions)
-    subscription = @target.subscriptions.find_by_user_id(@user.id)
+    subscription = @target.subscriptions.find { |subscription| subscription.user_id == @user.id }
     subscription && !subscription.subscribed
   end

app/models/user.rb

@@ -1038,7 +1038,10 @@ class User < ActiveRecord::Base
   def notification_settings_for(source)
     if notification_settings.loaded?
-      notification_settings.find { |notification| notification.source == source }
+      notification_settings.find do |notification|
+        notification.source_type == source.class.base_class.name &&
+          notification.source_id == source.id
+      end
     else
       notification_settings.find_or_initialize_by(source: source)
     end

changelogs/unreleased/n-plus-one-notification-recipients.yml

+---
+title: Fix some sources of excessive query counts when calculating notification recipients
+merge_request:
+author:
+type: performance

spec/services/notification_recipient_service_spec.rb

+require 'spec_helper'
+
+describe NotificationRecipientService do
+  let(:service) { described_class }
+  let(:assignee) { create(:user) }
+  let(:project) { create(:project, :public) }
+  let(:other_projects) { create_list(:project, 5, :public) }
+
+  describe '#build_new_note_recipients' do
+    let(:issue) { create(:issue, project: project, assignees: [assignee]) }
+    let(:note) { create(:note_on_issue, noteable: issue, project_id: issue.project_id) }
+
+    def create_watcher
+      watcher = create(:user)
+      create(:notification_setting, project: project, user: watcher, level: :watch)
+
+      other_projects.each do |other_project|
+        create(:notification_setting, project: other_project, user: watcher, level: :watch)
+      end
+    end
+
+    it 'avoids N+1 queries' do
+      create_watcher
+
+      service.build_new_note_recipients(note)
+
+      control_count = ActiveRecord::QueryRecorder.new do
+        service.build_new_note_recipients(note)
+      end
+
+      create_watcher
+
+      expect { service.build_new_note_recipients(note) }.not_to exceed_query_limit(control_count)
+    end
+  end
+end