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Dynamic nested forms using jQuery made easy; works with formtastic, simple_form or default forms
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cocoon is a Rails3 gem 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.

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"

Rails 3.1

Add the following to application.js so it compiles to the asset_pipeline

//= require cocoon

Rails 3.x

If you are using Rails 3.0.x, you need to run the installation task (since rails 3.1 this is no longer needed):

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, :reject_if => :all_blank, :allow_destroy => true

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.

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 four parameters:

  • name: the text to show in the link
  • f: referring to the containing form-object
  • association: the name of the association (plural) of which a new instance needs to be added (symbol or string).
  • html_options: extra html-options (see link_to) There are some special options, the first three allow to control the placement of the new link-data:
    • data-association-insertion-node : the jquery selector of the node
    • data-association-insertion-method : jquery method that inserts the new data. before, after, append, prepend, etc. Default: before
    • data-association-insertion-position : old method specifying where to insert new data.
      • this setting still works but data-association-insertion-method takes precedence. may be removed in a future version.
    • partial: explicitly declare the name of the partial that will be used
    • render_options : options passed through to the form-builder function (e.g. simple_fields_for, semantic_fields_for or fields_for). If it contains a :locals option containing a hash, that is handed to the partial.

Optionally you could also leave out the name and supply a block that is captured to give the name (if you want to do something more complicated).


Inside the html_options you can add an option :render_options, and the containing hash will be handed down to the form-builder for the inserted form. E.g. especially when using twitter-bootstrap and simple_form together, the simple_fields_for needs the option :wrapper => 'inline' which can be handed down as follows:

= link_to_add_association 'add something', f, :something, :render_options => {:wrapper => 'inline' }

If you want to specify locals that needed to handed down to the partial, write

= link_to_add_association 'add something', f, :something, :render_options => {:locals => {:sherlock => 'Holmes' }}


To overrule the default partial name, e.g. because it shared between multiple views, write

= link_to_add_association 'add something', f, :something, :partial => 'shared/something_fields'


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.

It takes three parameters:

  • name: the text to show in the link
  • f: referring to the containing form-object
  • html_options: extra html-options (see link_to)

Optionally you could also leave out the name and supply a block that is captured to give the name (if you want to do something more complicated).

Callbacks (upon insert and remove of items)

There is an option to add a callback on insertion or removal. If in your view you have the following snippet to select an owner (we use slim for demonstration purposes)

    = f.association :owner, :collection => Person.all(:order => 'name'), :prompt => 'Choose an existing owner'
  = link_to_add_association 'add a new person as owner', f, :owner

This view part will either let you select an owner from the list of persons, or show the fields to add a new person as owner.

The callbacks can be added as follows:

$(document).ready(function() {
         function() {
           $("#owner a.add_fields").hide();
         function() {
           $("#owner a.add_fields").show();
         function() {
           /* e.g. recalculate order of child items */

Do note that for the callbacks to work there has to be a surrounding container (div), where you can bind the callbacks to. Note that the default removal-callback is called before removing the nested item.

Control the Insertion behaviour

The default insertion location is at the back of the current container. But we have added two data-attributes that are read to determine the insertion-node and -method.

For example:

$(document).ready(function() {
    $("#owner a.add_fields").
      data("association-insertion-method", 'before').
      data("association-insertion-node", 'this');

The association-insertion-node will determine where to add it. You can choose any selector here, or specify this (default it is the parent-container).

The association-insertion-method will determine where to add it in relation with the node. Any jQuery DOM Manipulation method can be set but we recommend sticking to any of the following: before, after, append, prepend. It is unknown at this time what others would do.


If no explicit partial-name is given, cocoon looks for a file named _<association-object_singular>_fields. To override the default partial-name use the option :partial.

For the javascript to behave correctly, the partial should start with a container (e.g. 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.


  • add more sample relations: has_many :through, belongs_to, ...
  • improve the tests (test the javascript too)(if anybody wants to lend a hand ...?)


Copyright (c) 2010 Nathan Van der Auwera. See LICENSE for details.

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