Why polymorphic association doesn't work for STI if type column of the polymorphic association doesn't point to the base model of STI?

Question

I have a case of polymorphic association and STI here.

# app/models/car.rb
class Car < ActiveRecord::Base
  belongs_to :borrowable, :polymorphic => tru         


        

Answer 1

An older question, but the issue in Rails 4 still remains. Another option is to dynamically create/overwrite the _type method with a concern. This would be useful if your app uses multiple polymorphic associations with STI and you want to keep the logic in one place.

This concern will grab all polymorphic associations and ensure that the record is always saved using the base class.

# models/concerns/single_table_polymorphic.rb
module SingleTablePolymorphic
  extend ActiveSupport::Concern

  included do
    self.reflect_on_all_associations.select{|a| a.options[:polymorphic]}.map(&:name).each do |name|
      define_method "#{name.to_s}_type=" do |class_name|
        super(class_name.constantize.base_class.name)
      end
    end
  end
end

Then just include it in your model:

class Car < ActiveRecord::Base
  belongs_to :borrowable, :polymorphic => true
  include SingleTablePolymorphic
end