Fix destroying of users in relation to notifications

app/models/notification.rb

@@ -1,5 +1,5 @@
 class Notification < ActiveRecord::Base
-  has_many :user_notifications, inverse_of: :notification
+  has_many :user_notifications, inverse_of: :notification, dependent: :destroy
   has_many :users, through: :user_notifications
   belongs_to :generator_user, class_name: 'User'
 

app/models/user.rb

@@ -92,11 +92,14 @@ class User < ActiveRecord::Base
   has_many :restored_protocols, class_name: 'Protocol', foreign_key: 'restored_by_id', inverse_of: :restored_by
   has_many :user_notifications, inverse_of: :user
   has_many :notifications, through: :user_notifications
+
   # If other errors besides parameter "avatar" exist,
   # they will propagate to "avatar" also, so remove them
   # and put all other (more specific ones) in it
   after_validation :filter_paperclip_errors
 
+  before_destroy :destroy_notifications
+
   def name
     full_name
   end
@@ -251,4 +254,25 @@ class User < ActiveRecord::Base
       errors.add(:time_zone)
     end
   end
+
+  private
+
+  def destroy_notifications
+    # Find all notifications where user is the only reference
+    # on the notification, and destroy all such notifications
+    # (user_notifications are destroyed when notification is
+    # destroyed). We try to do this efficiently (hence in_groups_of).
+    nids_all = notifications.pluck(:id)
+    nids_all.in_groups_of(1000, false) do |nids|
+      Notification
+        .where(id: nids)
+        .joins(:user_notifications)
+        .group('notifications.id')
+        .having('count(notification_id) <= 1')
+        .destroy_all
+    end
+
+    # Now, simply destroy all user notification relations left
+    user_notifications.destroy_all
+  end
 end