How does Rails populate the “model_type” field for polymorphic associations?

问题

I have an Activity model. It belongs_to :parent, :polymorphic => true.

Does Rails use parent.class.name, parent.model_name or something else to populate the parent_type field?

I want a Presenter to behave like the parent object it wraps, and I need to override the right method.

Thanks.

回答1:

I'm working with Rails 3.0.7 right now, and the polymorphic type is being defined in active_record-3.0.7/lib/active_record/association.rb, line 1773.

def create_belongs_to_reflection(association_id, options)
  options.assert_valid_keys(valid_keys_for_belongs_to_association)
  reflection = create_reflection(:belongs_to, association_id, options, self)

  if options[:polymorphic]
    reflection.options[:foreign_type] ||= reflection.class_name.underscore + "_type"
  end

  reflection
end

So it looks like it is calling class_name.underscore and then appending "_type". This may be slightly different for rails 3.1, but this should be a good starting place.

来源：https://stackoverflow.com/questions/8350037/how-does-rails-populate-the-model-type-field-for-polymorphic-associations