Question

I am developing a controller that creates a model with a polymorphic belongs_to association. What I do now to find the model it belongs to is as follows:

 def find_polymorphic_model(classes)
   classes_names = classes.map { |c| c.name.underscore + '_id' }

   params.select { |k, v| classes_names.include?(k) }.each do |name, value|
     if name =~ /(.+)_id$/
       return $1.classify.constantize.find(value)
     end
   end

  raise InvalidPolymorphicType
end

Where classes is an array of valid types for the association.

The problem with this approach is that I have to remember in the controller which types are allowed for the model I am creating.

Is there a way to find which types are allowed for a certain polymorphic belongs_to association? Or maybe I am doing this wrong and I should not let a polymorphic controller be exposed without nesting it in the polymorphic resource (in the router)?

I also think there may be problems with the fact that Rails lazy loads classes, so to be able to find out this thing I would have to explicitly load all models at initialization time.

Answer 1

For your validation you don’t have to get all the possible polymorphic types. All you need is to check if the specified type (say, the value of taggable_type attribute) is suitable. You can do it this way:

# put it in the only_polymorphic_validator.rb. I guess under app/validators/. But it's up to you.
class OnlyPolymorphicValidator < ActiveModel::EachValidator
    def validate_each(record, attribute, value)
        polymorphic_type = attribute.to_s.sub('_type', '').to_sym
        specified_class = value.constantize rescue nil
        this_association = record.class.to_s.underscore.pluralize.to_sym

        unless(specified_class.reflect_on_association(this_association).options[:as] == polymorphic_type rescue false)
            record.errors[attribute] << (options[:message] || "isn't polymorphic type")
        end
    end
end

And then use:

validates :taggable_type, only_polymorphic: true

to check whether :taggable_type contains a valid class.