class ActiveRecord::HasManyThroughAssociationPolymorphicSourceError

def initialize(owner_class_name, reflection, source_reflection)
  super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}' without 'source_type'. Try adding 'source_type: \"#{reflection.name.to_s.classify}\"' to 'has_many :through' definition.")
end