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

[Andrew Havens]

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!