Simple example showing how to safely add a method to ActionView::Helpers::FormBuilder

form_helper.rb

module FormHelper
  class CustomFormBuilder < ActionView::Helpers::FormBuilder
    def autocomplete_field(method, url, options = {})
      text_field(method, options.reverse_merge(:'data-autocomplete' => url)
    end
  end

  %w(form_for fields_for).each do |method|
    class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
      def omega_#{method}(record_or_name_or_array, *args, &proc)
        options = args.extract_options!
        options[:builder] = CustomFormBuilder
        #{method}(record_or_name_or_array, *(args << options), &proc)
    end
    RUBY_EVAL
  end
end