now, collection can add and remove learning objects

app/models/collection.rb

@@ -10,6 +10,31 @@ class Collection
     super(params.merge(defaults))
   end
 
+  ##
+  # Add +learning_object+ to collection
+  #
+  # After adding some learning object to collection, you must use CollectionRepository to persist
+  # the changes.
+  #
+  # Example:
+  #   @test_collection = Collection.new(name: 'test', description: 'test', owner: current_user)
+  #   @test_collection.add @learning_object
+  #
+  #   @test_collection.learning_objects # [@learning_object]
+  #   collection_repository.save @test_collection
+  def add(learning_object)
+    learning_objects.append learning_object
+  end
+
+  ##
+  # Remove +learning_object+ from collection
+  #
+  # After remove some learning object to collection, you must use CollectionRepository to persist
+  # the changes.
+  def remove(learning_object)
+    learning_objects.delete learning_object
+  end
+
   private
 
   def defaults