gistfile1.txt

#config/initializers/application_form_builder.rb:

ActionView::Base.class_eval do
  def self.default_form_builder
    Forms::ApplicationFormBuilder
  end
end


#helpers/organisations_helper.rb:

class OrganisationsHelper
  # ...elided...
end

Forms::ApplicationFormBuilder.graft(OrganisationsHelper)


#lib/forms/application_form_builder.rb:

# Customised FormBuilder which adds the ability to customise its methods.
# The actual customisations are in module Forms::ApplicationHelper.
class Forms::ApplicationFormBuilder < ActionView::Helpers::FormBuilder

  # Grafts methods in a module onto the helper so that they can be called from a template.
  def self.graft(grafted_module)
    # Copied from FormBuilder. FormBuilder looks like it has some bright
    # engineering ideas but never finished implementing them. This *should*
    # be automated by defining "self.field_helpers", but it's used before
    # this class is loaded.
    grafted_module.instance_methods.each do |selector|
      src = <<-end_src
        def #{selector}(method, options = {})
          @template.send(#{selector.inspect}, @object_name, method, objectify_options(options))
        end
      end_src
      class_eval src, __FILE__, __LINE__
    end
  end

  # This particular helper we graft on always.
  graft(Forms::ApplicationHelper)

  private

  def objectify_options(options)
    @default_options.merge(options.merge(:object => @object))
  end
end


#lib/forms/application_helper.rb:

module Forms::ApplicationHelper
  # date_field helper, delegate to the one we already have.
  def date_field(object_name, method, options = {})
    super(object_name, method, options)
  end
end