validates uniqueness of review for user/reviewable

app/models/review.rb

@@ -10,6 +10,7 @@ class Review < ActiveRecord::Base
 
   validates_presence_of :user, :reviewable
   validates_inclusion_of :reviewable_type, in: %w(LearningObject Collection), message: 'Only LearningObjects and Collections are reviewable.'
+  validates_uniqueness_of :user, scope: [:reviewable_id, :reviewable_type]
 
   default_scope { includes(:user) }