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+h2. 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.
+
+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 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:
+
+<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]" 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| %>
+    <%= af.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]" type="text" />
+
+  <input id="person_address_attributes_street" name="person[address_attributes][street]" type="text" />
+</form>
+</html>
+
+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:
+
+<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| %>
+    <%= pf.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]" 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>
+</html>
+
+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:
+
+<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.