README.rdoc

Custom Form Builder

The idea is to create a custom form builder, which always adds a label to a field. Then you don’t have to add the label specifically. This is taken from Chad Fowler, Rails Recipes, 2012. (Chapter 32)

The interesting bit is the following metaprogramming class, which is homed in the app/helpers directory.

class LabeledFormBuilder < ActionView::Helpers::FormBuilder

(field_helpers -
  %w(check_box radio_button hidden_field label)).each do |selector|
    src = <<-END_SRC
      def #{selector}(field, options = {})
        @template.content_tag("p", label(field) + ": " + super,
         :class => @template.cycle("", "alt-row"))
      end
    END_SRC
    class_eval src, __FILE__, __LINE__
end

end

and then do something like .alt-row { background: #fab444; }

The inner loop generates some code, which is then class_eval’d to incorporate it into the FormBuilder as new code. That is, to create new definitions of things like check_box and label.

A number of questions arise about this:

1) What is the dash symbol doing in the line with field_helpers? 2) What do __FILE__ and __LINE__ do? 3) what is the super token doing here?

One answer to the first question: The dash removes from the array. But, if we are defining these, why would we want them removed from ‘field_helpers’?

__FILE__ is current relative path __LINE__ is the current line These last 2 items are added to be able to provide correct file name and correct line number.

Fowler also notes that you can do an initializer, config/initializers/ custom_form_builder.rb:

require ‘labeled_form_builder’ ActionView::Base.default_form_builder = LabeledFormBuilder

to always get it. All form_for calls will then get this one.