Rails Nested Model Forms

Creating a form for a model and its associations can become quite tedious. Therefore Rails provides helpers to assist in dealing with the complexities of generating these forms and the required CRUD operations to create, update, and destroy associations.

After reading this guide, you will know:

• do stuff.

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NOTE: This guide assumes the user knows how to use the {Rails form helpers} in general. Also, it's not an API reference. For a complete reference please visit the {Rails API documentation}.

Model setup

To be able to use the nested model functionality in your forms, the model will need to support some basic operations.

First of all, it needs to define a writer method for the attribute that corresponds to the association you are building a nested model form for. The fields_for form helper will look for this method to decide whether or not a nested model form should be built.

If the associated object is an array, a form builder will be yielded for each object, else only a single form builder will be yielded.

Consider a Person model with an associated Address. When asked to yield a nested FormBuilder for the :address attribute, the fields_for form helper will look for a method on the Person instance named address_attributes=.

ActiveRecord::Base model

For an ::ActiveRecord::Base model and association this writer method is commonly defined with the accepts_nested_attributes_for class method:

has_one

class Person < ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address
end

belongs_to

class Person < ActiveRecord::Base
  belongs_to :firm
  accepts_nested_attributes_for :firm
end

has_many / has_and_belongs_to_many

class Person < ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects
end

NOTE: For greater detail on associations see Active Record Associations. For a complete reference on associations please visit the API documentation for ActiveRecord::Associations::ClassMethods.

Custom model

As you might have inflected from this explanation, you don't necessarily need an ActiveRecord::Base model to use this functionality. The following examples are sufficient to enable the nested model form behavior:

Single associated object

class Person
  def address
    Address.new
  end

  def address_attributes=(attributes)
    # ...
  end
end

Association collection

class Person
  def projects
    [Project.new, Project.new]
  end

  def projects_attributes=(attributes)
    # ...
  end
end

NOTE: See (TODO) in the advanced section for more information on how to deal with the CRUD operations in your custom model.

Views

Controller code

A nested model form will only be built if the associated object(s) exist. This means that for a new model instance you would probably want to build the associated object(s) first.

Consider the following typical RESTful controller which will prepare a new Person instance and its address and projects associations before rendering the new template:

class PeopleController < ApplicationController
  def new
    @person = Person.new
    @person.built_address
    2.times { @person.projects.build }
  end

  def create
    @person = Person.new(params[:person])
    if @person.save
      # ...
    end
  end
end

NOTE: Obviously the instantiation of the associated object(s) can become tedious and not DRY, so you might want to move that into the model itself. ActiveRecord::Base provides an after_initialize callback which is a good way to refactor this.

Form code

Now that you have a model instance, with the appropriate methods and associated object(s), you can start building the nested model form.

Standard form

Start out with a regular RESTful form:

<%= form_for @person do |f| %>
  <%= f.text_field :name %>
<% end %>

This will generate the following html:

<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" type="text" />
</form>

Nested form for a single associated object

Now add a nested form for the address association:

<%= form_for @person do |f| %>
  <%= f.text_field :name %>

  <%= f.fields_for :address do |af| %>
    <%= af.text_field :street %>
  <% end %>
<% end %>

This generates:

<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" type="text" />

  <input id="person_address_attributes_street" name="person[address_attributes][street]" type="text" />
</form>

Notice that fields_for recognized the address as an association for which a nested model form should be built by the way it has namespaced the name attribute.

When this form is posted the Rails parameter parser will construct a hash like the following:

{
  "person" => {
    "name" => "Eloy Duran",
    "address_attributes" => {
      "street" => "Nieuwe Prinsengracht"
    }
  }
}

That's it. The controller will simply pass this hash on to the model from the create action. The model will then handle building the address association for you and automatically save it when the parent (person) is saved.

Nested form for a collection of associated objects

The form code for an association collection is pretty similar to that of a single associated object:

<%= form_for @person do |f| %>
  <%= f.text_field :name %>

  <%= f.fields_for :projects do |pf| %>
    <%= pf.text_field :name %>
  <% end %>
<% end %>

Which generates:

<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" type="text" />

  <input id="person_projects_attributes_0_name" name="person[projects_attributes][0][name]" type="text" />
  <input id="person_projects_attributes_1_name" name="person[projects_attributes][1][name]" type="text" />
</form>

As you can see it has generated 2 project name inputs, one for each new project that was built in the controller's new action. Only this time the name attribute of the input contains a digit as an extra namespace. This will be parsed by the Rails parameter parser as:

{
  "person" => {
    "name" => "Eloy Duran",
    "projects_attributes" => {
      "0" => { "name" => "Project 1" },
      "1" => { "name" => "Project 2" }
    }
  }
}

You can basically see the projects_attributes hash as an array of attribute hashes, one for each model instance.

NOTE: The reason that fields_for constructed a hash instead of an array is that it won't work for any form nested deeper than one level deep.

TIP: You can however pass an array to the writer method generated by accepts_nested_attributes_for if you're using plain Ruby or some other API access. See (TODO) for more info and example.