Custom Rails Form Builders

I don't think it's intended that anyone build their large scale application with only using the default Rails form builder. There are ways to get around some of the annoyances. Several people have built libraries dedicated to giving you a better API on top of the default. At Brand New Box we've used Simple Form quite a bit in the past for it's power and flexibility. It allowed us to abstract the structure of our forms and have a single place we could update that. That did come at a bit of a cost as our whole team had to learn the custom DSL that simple form provies you with and anytime that we onboard someone we also had to teach them.

= simple_form_for @user do |f|
  = f.input :username
  = f.input :password

However Rails gives us another ability, we can customize the default form builder that is used to build the forms. In fact, this is the technique that libraries like Simple Form and Formtastic use! They provide a custom form builder which is what you are using when you are calling methods like f.input .

We figured out that we could use that same technique and write our own custom form builder that was all of our own code.

= form_with model: @user, builder: AppFormBuilder do |f|
  .form-group
    = f.label :username
    = f.text_field :username

  .form-group
    = f.label :password
    = f.text_field :password

When using the builder argument Rails will yield an instance of the class that is passed in to the form builder. On that custom class we can add whatever methods that we want.

This is no doubt a very dense 211 lines of code. But the advantage here is that this code lives in our repository and it's normal Rails/Ruby code. Anybody on the team can edit it without having to learn a custom DSL.

Some features that we have built into this custom form builder include

• f.input which generates a label and input for a given method of a model.
• The proper input type is looked up based on the backing database column (it uses "text" inputs for string columns, "number" inputs for numeric columns, etc). We even try to detect if the field for the input might be a password field (by looking if it contains the word "password") and render a password input! That can also be customized by using the "as" argument to input.

  = f.input :description, as: :text

• Automatic support of collection inputs with f.collection ,
• The structure of our inputs is able to be modified in a single place by editing the form_group method in the form builder.
• Error messages are automatically added to our inputs.
• Hints are supported with the "hint" attribute.

None of these features came for free, they all had to be built. But any developer on our team can look through the code and see how those things got there.

With all of those advantages we can now write the the previous form as follows.

= form_with model: @user, builder: AppFormBuilder do |f|
  = f.input :username
  = f.input :password

We even often make a special helper for generating a form using our custom builder.

def app_form_for(name, *args, &block)
  options = args.extract_options!
  args << options.merge(builder: AppFormBuilder)
  form_for(name, *args, &block)
end

def app_form_with(model: nil, scope: nil, url: nil, format: nil, **options, &block)
  options = options.reverse_merge(builder: AppFormBuilder)
  form_with(model: model, scope: scope, url: url, format: format, **options, &block)
end

Now that we have built out that custom form builder we copy it to each of our projects and make per projects customizations to it. The workflow has worked very well for our team. It has given us the right amount of power and flexibility while still being able to onboard new team members quickly.