h2. Rails nested model forms

Creating a form for a model _and_ its associations can become quite tedious. Therefor 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.

In this guide you will:

* do stuff

endprologue.

NOTE: This guide assumes the user knows how to use the "Rails form helpers":form_helpers.html in general. Also, it’s *not* an API reference. For a complete reference please visit "the Rails API documentation":http://api.rubyonrails.org/.


h3. 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 build.

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=+.

h4. ActiveRecord::Base model

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

h5. has_one

<ruby>
class Person < ActiveRecord::Base
  has_one :address
  accepts_nested_attributes_for :address
end
</ruby>

h5. belongs_to

<ruby>
class Person < ActiveRecord::Base
  belongs_to :firm
  accepts_nested_attributes_for :firm
end
</ruby>

h5. has_many / has_and_belongs_to_many

<ruby>
class Person < ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects
end
</ruby>

h4. 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 behaviour:

h5. Single associated object

<ruby>
class Person
  def address
    Address.new
  end
  
  def address_attributes=(attributes)
    # ...
  end
end
</ruby>

h5. Association collection

<ruby>
class Person
  def projects
    [Project.new, Project.new]
  end
  
  def projects_attributes=(attributes)
    # ...
  end
end
</ruby>

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

h3. Views

h4. Controller code

A nested model form will _only_ be build 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:

<ruby>
class PeopleController < ActionController:Base
  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
</ruby>

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.

h4. 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.

h5. Standard form

Start out with a regular RESTful form:

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

This will generate the following html:

<html>
<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" size="30" type="text" />
</form>
</html>

h5. Nested form for a single associated object

Now add a nested form for the +address+ association:

<erb>
<% form_for @person do |f| %>
  <%= f.text_field :name %>
  
  <% f.fields_for :address do |af| %>
    <%= f.text_field :street %>
  <% end %>
<% end %>
</erb>

This generates:

<html>
<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" size="30" type="text" />
  
  <input id="person_address_attributes_street" name="person[address_attributes][street]" size="30" type="text" />
</form>
</html>

Notice that +fields_for+ recognized the +address+ as an association for which a nested model form should be build 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:

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

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.

h5. 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:

<erb>
<% form_for @person do |f| %>
  <%= f.text_field :name %>
  
  <% f.fields_for :projects do |pf| %>
    <%= f.text_field :name %>
  <% end %>
<% end %>
</erb>

Which generates:

<html>
<form action="/people" class="new_person" id="new_person" method="post">
  <input id="person_name" name="person[name]" size="30" type="text" />
  
  <input id="person_projects_attributes_0_name" name="person[projects_attributes][0][name]" size="30" type="text" />
  <input id="person_projects_attributes_1_name" name="person[projects_attributes][1][name]" size="30" type="text" />
</form>
</html>

As you can see it has generated 2 +project name+ inputs, one for each new +project+ that’s build in the controllers +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:

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

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 form which would result in a hash instead of an array is that it won't work for any forms 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.