Rails polymorphic / single-table-inheritance models, select type in form

Question

Update: After thinking about this some more I think this question includes both polymorphism and STI.

Let's say I have the following:

class Vehicle < ActiveRecord::Base
  belongs_to :dealership
  ...
end

class Car < Vehicle
  ...
end

class Truck < Vehicle
  ...
end

The Car and Truck models have the same attributes but different logic in the model, so they all just use the Vehicles table.

I'd like to make a generic form that allows someone to create a new vehicle record, and to select whether it is a "Car" or "Truck". On the backend this form should then create a "Car" or "Truck" record, not a generic "Vehicle" record.

Question: How do you present this option in a Rails form? Specifically, are there methods for listing the child types of an abstract base model, and/or is there a simple way to present these as (for instance) a Select element in a form?

The reason I am looking to create a form that can make a car or a truck opposed to just having two different forms for cars and trucks, is eventually I'd also like these models to be nested in a parent, "Dealership." I'd like to be able to not only create new vehicles and select "car" or "truck" as an option, but to be able to have nested forms where a user could load a "Dealership" and view/edit all the cars and trucks that belong to that dealership in a single form. I know how to set that up using accepts_nested_attributes_for on a simple association like Vehicle belongs_to :dealership, but I'm not sure how to set it up for child models that inherit from Vehicle.

Anton Evangelatov Anton Evangelatov · Accepted Answer · 2014-05-07T21:27:38

I've had the same problem. Basically you are trying to build a form that accepts has_many association of a base class, but you want to create individual forms (add nested fields) for any of the Decorated / Concrete classes dynamically.

I solved this using the cocoon gem, which provided methods that dynamically add the correct fields to the form. The _form partial looks something like:

# _form.html.haml

= simple_form_for @dealership, :html => { :multipart => true } do |f|

  = f.simple_fields_for :vehicles do |vehicle|
    = render 'vehicle_fields', :f => vehicle
    = link_to_add_association 'Add a Car', f, :vehicles, :wrap_object => Proc.new { |vehicle| vehicle = Car.new }
    = link_to_add_association 'Add a Truck', f, :vehicles, :wrap_object => Proc.new { |vehicle| vehicle = Truck.new }

= f.button :submit, :disable_with => 'Please wait ...', :class => "btn btn-primary", :value => 'Save'

And then the _vehicle_fields

# _vehicle_fields.html.haml

- if f.object.type == 'Car'
  = render 'car_fields', :f => f
- elsif f.object.type == 'Truck'
  = render 'truck_fields', :f => f

car_fields and truck_fields would also feature a link link_to_remove_association provided by cocoon

Take a look at :wrap_object documentation at https://github.com/nathanvda/cocoon , or read my blog post at http://www.powpark.com/blog/programming/2014/05/07/rails_nested_forms_for_single_table_inheritance_associations

Rails polymorphic / single-table-inheritance models, select type in form

3 Answers