enables notification to be extended

app/models/notification.rb

@@ -3,7 +3,7 @@ class Notification < ActiveRecord::Base
   has_many :users, through: :user_notifications
   belongs_to :generator_user, class_name: 'User'
 
-  enum type_of: [:assignment, :recent_changes, :system_message]
+  enum type_of: Constants::NOTIFICATIONS_TYPES
 
   def already_seen(user)
     UserNotification.where(notification: self, user: user)

config/initializers/constants.rb

@@ -196,6 +196,10 @@ class Constants
     'text/plain'
   ].freeze
 
+  # Notification types. Should not be freezed, as modules might append to this.
+  NOTIFICATIONS_TYPES = { assignment: 0,
+                          recent_changes: 1,
+                          system_message: 2 }
   # Organization name for default admin user
   DEFAULT_PRIVATE_ORG_NAME = 'My projects'.freeze