fix regression in visible notification detection

app/models/notification.rb

@@ -9,7 +9,8 @@ class Notification < ActiveRecord::Base
 
   scope :unread, lambda { where(read: false) }
   scope :recent, lambda { |n=nil| n ||= 10; order('notifications.created_at desc').limit(n) }
-  scope :visible , lambda { joins('LEFT JOIN topics ON notifications.topic_id = topics.id AND topics.deleted_at IS NULL') }
+  scope :visible , lambda { joins('LEFT JOIN topics ON notifications.topic_id = topics.id')
+                            .where('topics.id IS NULL OR topics.deleted_at IS NULL') }
 
   after_save :refresh_notification_count
   after_destroy :refresh_notification_count

spec/models/notification_spec.rb

@@ -277,6 +277,13 @@ describe Notification do
       fab(Notification.types[:liked], true)
     end
 
+    it 'correctly finds visible notifications' do
+      pm
+      expect(Notification.visible.count).to eq(1)
+      post.topic.trash!
+      expect(Notification.visible.count).to eq(0)
+    end
+
     it 'orders stuff correctly' do
       a = unread_pm
           regular