Ruby on Rails

Here's a simplified example of what I have so far:

class Garage    include Mongoid::Document    embeds_one :vehicle    accepts_nested_attributes_for :vehicle  end    class Vehicle    include Mongoid::Document    embedded_in :garage    attr_accessible :_type  end    class Car < Vehicle    field :car_field    attr_accessible :car_field  end    class Truck < Vehicle    field :truck_field    attr_accessible :truck_field  end

In the console:

> garage = Garage.new  > garage.vehicle = Car.new(car_field: 'something')  > garage.save!

In the form:

= simple_form_for @garage do |f|    = f.input :vehicle do |vehicle_form|       = vehicle_form.input :_type, collection: ['Car', 'Truck']      %span.hide{data:{fields-for:'Car'}}      = vehicle_form.input :car_field      %span.hide{data:{fields-for:'Truck'}}      = vehicle_form.input :truck_field    :coffeescript    $('#garage_vehicle_attributes__type').change ->      type = $(@).find('option:selected').val()      $('[data-fields-for="' + type + '"]').show()

The problem that will occur in this example is that it won't be able to render the truck_field becauseCar does not have a truck_field method. I'm not sure how to solve this problem besides throwing out any form helpers and managing the html and field values manually. Even after much Googling, I haven't been able to find any examples of this type of form.

How can this problem be solved in a standard, "Rails way" using existing form helpers?

Any tips or advice you can give would be greatly appreciated. Thanks!