Extension to formtastic to make dynamic nested forms easier, using jQuery
Ruby JavaScript



Note: this gem is deprecated. The gem has been renamed to cocoon, as it was not limited to formtastic alone.

Formtastic-cocoon is a Rails3 gem, extending formtastic, to allow easier handling of nested forms.

Nested forms are forms that handle nested models and attributes in one form. For example a project with its tasks, an invoice with its ordered items.


As this gem extends formtastic and uses jQuery, it is only useful to use this gem in a rails3 project where you are already using formtastic and jQuery.

I have a sample project where I demonstrate both.


Inside your Gemfile add the following:

gem "formtastic_cocoon"

Run the installation task:

rails g formtastic_cocoon:install

This will install the needed javascript file. Inside your application.html.haml you will need to add below the default javascripts:

= javascript_include_tag :formtastic_cocoon

or using erb, you write

<%= javascript_include_tag :formtastic_cocoon %>

That is all you need to do to start using it!


Suppose you have a model Project:

rails g scaffold Project name:string description:string

and a project has many tasks:

rails g model Task description:string done:boolean project_id:integer

Edit the models to code the relation:

class Project < ActiveRecord::Base
  has_many :tasks
  accepts_nested_attributes_for :tasks

class Task < ActiveRecord::Base
  belongs_to :project

What we want to achieve is to get a form where we can add and remove the tasks dynamically. What we need for this, is that the fields for a new/existing task are defined in a partial view called _task_fields.html.

Inside our projects/_form partial we then write:

- f.inputs do
  = f.input :name
  = f.input :description
  %h3 Tasks
    = f.semantic_fields_for :tasks do |task|
      = render 'task_fields', :f => task
      = link_to_add_association 'add task', f, :tasks
  -f.buttons do
    = f.submit 'Save'

and inside the _task_fields partial we write:

  = f.inputs do
    = f.input :description
    = f.input :done, :as => :boolean
    = link_to_remove_association "remove task", f

That is all there is to it!

There is an example project on github implementing it called formtastic-cocoon-demo.

How it works

I define two helper functions:


This function will add a link to your markup that will, when clicked, dynamically add a new partial form for the given association. This should be placed below the semantic_fields_for.

It takes three parameters:

  • name: the text to show in the link
  • f: referring to the containing formtastic form-object
  • association: the name of the association (plural) of which a new instance needs to be added (symbol or string).


This function will add a link to your markup that will, when clicked, dynamically remove the surrounding partial form. This should be placed inside the partial _<association-object-singular>_fields.


The partial should be named _<association-object_singular>_fields, and should start with a div of class .nested-fields.

There is no limit to the amount of nesting, though.

Note on Patches/Pull Requests

  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.


Copyright (c) 2010 nathanvda. See LICENSE for details.