cocoon is a Rails3 gem to allow easier handling of nested forms.

Nested forms are forms that handle nested models and attributes dynamically in one form.
Some standard examples: a project with its tasks, an invoice with its ordered items.

It is formbuilder-agnostic, so it works with standard Rails, or formtastic or simple_form.


This gem uses jQuery, it is most useful to use this gem in a rails3
project where you are already using jQuery.

Furthermore i would advice you to use either formtastic or simple_form.

I have a sample project where I demonstrate the use of cocoon with formtastic.


Inside your `Gemfile` add the following:

    gem "cocoon"

Run the installation task:

    rails g 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 :cocoon

or using erb, you write

    <%= javascript_include_tag :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.

Note: the name of the partial is really important, as this plugin will look for the partial named as the singular of the relation + _fields.

We will show the sample usage with the different possible form-builders.

Using formtastic

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 cocoon-formtastic-demo.

Using simple_form

This is almost identical to formtastic, instead of writing semantic_fields_for you write simple_fields_for.

There is an example project on github implementing it called cocoon_simple_form_demo.

Using standard rails forms

I will provide a full example (and a sample project) later.

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.

To conclude

I hope it can be of use. Let me know what you think.