Add "include ActiveModel::Model"

app/models/learning_object.rb

 class LearningObject < OrientDb::LearningObjectRepository
 
+  include ActiveModel::Model
+
   attr_accessor :id, :name, :title, :author_id, :description, :date, :exercises,
                 :references, :supplementary_material, :license_name, :license_uri,
                 :files