rails activeform - put accepts_nested_attributes_for in its place

big balls of mud killer
, in 02 December 2014

I don’t know about you, but the amount of successes I had with accepts_nested_attributes_for were few and far between. It always felt like a clunky API. If you’ve ever worked on a large rails project you also know the benefits of giving a form its own proper class, instead of interspersing logic between the view, controller, and model.

I started rolling my own form objects using ActiveModel. While that is pretty nice, you have to remember how to make one each time, and there are lots of ActiveModel conventions you need to follow.

I recently discovered active_form, which appears to be a gsoc project to tackle this problem. Its the nicest abstraction around forms that I have found to date.

You can create a new file under app/forms for your form. Say we want to create a ‘user’ model and a ‘subscription’ model on sign up, for stripe for example.

class SignupForm < ActiveForm::Base
  self.main_model = :user
  attributes :email, :password
  validates :email, :password, presence: true

  association :subscription do
    attributes :plan_id, :stripe_card_token, required: true

Then in the controller you initialize a new instance of the form class passing an instance of the main model for it to work on, and then call the #submit and #save methods on it.

user = User.new
@signup_form = SignupForm.new(User.new)
@signup_form.submit user_params

The associations will be saved automatically. If there are any validation errors on the main model of the associations they will be added to the form class.

Not only is this easier than rolling your own form class or using accepts_nested_attributes_for, I think this library can also encourage better modular design. You can create specific form classes for specific screens, instead of spreading logic all over your model just because you have one instance where you need to save two or more models with one form.

I’m not sure if this is slated for rails 5 at this point? The github repo is currently under the rails organization on github.


Since this post, activeform(now renamed to actionform) is actively being discouraged from real use. In reality it was only an experiment started in a GSOC project.

Personally I still prefer to use form objects in certain situations. In my own projects I have removed actionform and replaced my form objects with plain objects that mix in ActiveModel::Model. It is a bit similar to the way it is done in this thoughtbot post.