Rails Form helper to ease the rendering of nested_forms via accepts_nested_attributes_for

gistfile1.rb

module KingForm

  # Method for building forms that contain fields for associated models.
  module NestedFormHelper
    
    # Renders the form for nested objects defined via activerecord accepts_nested_attributes_for
    #
    # The associated argument can be either an object, or a collection of objects to be rendered.
    #
    # An options hash can be specified to override the default behaviors.
    # 
    # Options are:
    # * <tt>:new</tt>        - specify a certain number of new elements to be added to the form. Useful for displaying a 
    #                         few blank elements at the bottom.
    # * <tt>:name</tt>       - override the name of the association, both for the field names, and the name of the partial
    # * <tt>:partial</tt>    - specify the name of the partial in which the form is located.
    # * <tt>:fields_for</tt> - specify additional options for the fields_for_associated call
    # * <tt>:locals</tt>     - specify additional variables to be passed along to the partial
    # * <tt>:render</tt>     - specify additional options to be passed along to the render :partial call
    # * <tt>:skip</tt>       - array of elements which will be skipped, usefull if you already rendered a partial in the same form with parts of the data.
    #                       eg. obj.addresses, render the firt address on top of form, render all the other addresses at the bottom
    #
    def render_nested_form(associated, opts = {})
      associated = associated.is_a?(Array) ? associated : [associated] # preserve association proxy if this is one      
      opts.symbolize_keys!
      (opts[:new] - associated.select(&:new_record?).length).times { associated.build } if opts[:new]

      unless associated.empty?
        name              = extract_option_or_class_name(opts, :name, associated.first)
        partial           = opts[:partial] || name
        if opts[:skip]  # objects to be skipped are present
          skip_el = opts[:skip].is_a?(Array) ? opts[:skip] : [opts[:skip]]
          assoc_el = []
          associated.each { |el|  assoc_el << el unless skip_el.include?(el) }
        else # normal procedure
          assoc_el = associated
        end

        output = assoc_el.map do |element|
          fields_for(association_name(name), element, (opts[:fields_for] || {}).merge(:name => name)) do |f|

            @template.render( {:partial => "#{partial}",
                               #The current objects classname is always present in partial so:
                               #when Object is LineItem locals has :line_item => object
                               :locals => {name.to_sym => f.object, :f => f}.merge( opts[:locals] || {} )
                              }.merge( opts[:render] || {} ) )
          end
        end
        output.join
      end
    end
    
    private
      def association_name(class_name)
        @object.respond_to?("#{class_name}_attributes=") ? class_name : class_name.pluralize
      end
      def extract_option_or_class_name(hash, option, object)
        (hash.delete(option) || object.class.name.split('::').last.underscore).to_s
      end
  end
end


# include the nested_form helper method on the very top of FormBuilder
ActionView::Helpers::FormBuilder.class_eval { include KingForm::NestedFormHelper }

=begin

== Example Nested Forms

Put the above module and helper include somewhere in your rails instance (plugin/lib / ..)

The first thing you need to do is enable attributes on the association in the model.
  
  class Project < ActiveRecord::Base
    has_many :tasks
    accepts_nested_attributes_for :tasks
  end

The partial for the nested object(one Task) should have the name of the associated element's type( or the partial to use)

  ## projects/_task.html.erb
  <div class="task">
    <label>Title</label>
    <%= f.text_field :title %>
  </div>

In the parent element's form (Project), call the render_nested_form method, with the collection of
elements you'd like to render as the first argument.

  ## projects/_form.html.erb
  <%= f.render_nested_form(@project.tasks) %>

That call will render the partial named _task.html.erb with each element in the supplied collection of tasks, wrapping
the partial in a form builder (fields_for) with all the necessary arguments to produce a hash that will satisfy the
tasks_attributes method(provided by Rails new accepts_nested_attributes_for).

=== Options
See comments in nested_form_helper module for full reference of options.
You may want to add a few blank tasks to the bottom of your form; no need to do that in the controller anymore.

  <%= f.render_nested_form(@project.tasks, :new => 3, :partial=>'some_partial', :locals=>..) %>
=end