[Rails] Creating a nested attributes form for polymorphic/single table inheritance associations

I am working on a form (using SimpleForm) that allows you to edit embedded associations. The problem that I'm running into is that the nested models are subclasses so they are different types with potentially different fields. I'm creating hidden forms for each type of model, and using JavaScript to display the form for the selected type. I'm using Mongoid and SimpleForm, but I expect the implementation would be the same using ActiveRecord and the standard Rails form helpers.

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!

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