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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.
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 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=
.
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
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:
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 < 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
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 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.