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225 lines
6.8 KiB
Markdown
225 lines
6.8 KiB
Markdown
Rails nested model forms
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========================
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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.
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After reading this guide, you will know:
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* do stuff.
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--------------------------------------------------------------------------------
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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/).
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Model setup
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-----------
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To be able to use the nested model functionality in your forms, the model will need to support some basic operations.
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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.
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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.
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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=`.
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### ActiveRecord::Base model
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For an ActiveRecord::Base model and association this writer method is commonly defined with the `accepts_nested_attributes_for` class method:
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#### has_one
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```ruby
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class Person < ActiveRecord::Base
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has_one :address
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accepts_nested_attributes_for :address
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end
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```
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#### belongs_to
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```ruby
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class Person < ActiveRecord::Base
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belongs_to :firm
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accepts_nested_attributes_for :firm
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end
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```
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#### has_many / has_and_belongs_to_many
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```ruby
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class Person < ActiveRecord::Base
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has_many :projects
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accepts_nested_attributes_for :projects
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end
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```
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### Custom model
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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:
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#### Single associated object
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```ruby
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class Person
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def address
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Address.new
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end
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def address_attributes=(attributes)
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# ...
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end
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end
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```
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#### Association collection
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```ruby
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class Person
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def projects
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[Project.new, Project.new]
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end
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def projects_attributes=(attributes)
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# ...
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end
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end
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```
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NOTE: See (TODO) in the advanced section for more information on how to deal with the CRUD operations in your custom model.
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Views
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-----
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### Controller code
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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.
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Consider the following typical RESTful controller which will prepare a new Person instance and its `address` and `projects` associations before rendering the `new` template:
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```ruby
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class PeopleController < ApplicationController
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def new
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@person = Person.new
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@person.built_address
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2.times { @person.projects.build }
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end
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def create
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@person = Person.new(params[:person])
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if @person.save
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# ...
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end
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end
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end
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```
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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.
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### Form code
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Now that you have a model instance, with the appropriate methods and associated object(s), you can start building the nested model form.
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#### Standard form
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Start out with a regular RESTful form:
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```erb
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<%= form_for @person do |f| %>
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<%= f.text_field :name %>
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<% end %>
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```
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This will generate the following html:
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```html
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<form action="/people" class="new_person" id="new_person" method="post">
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<input id="person_name" name="person[name]" type="text" />
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</form>
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```
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#### Nested form for a single associated object
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Now add a nested form for the `address` association:
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```erb
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<%= form_for @person do |f| %>
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<%= f.text_field :name %>
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<%= f.fields_for :address do |af| %>
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<%= af.text_field :street %>
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<% end %>
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<% end %>
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```
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This generates:
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```html
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<form action="/people" class="new_person" id="new_person" method="post">
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<input id="person_name" name="person[name]" type="text" />
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<input id="person_address_attributes_street" name="person[address_attributes][street]" type="text" />
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</form>
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```
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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.
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When this form is posted the Rails parameter parser will construct a hash like the following:
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```ruby
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{
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"person" => {
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"name" => "Eloy Duran",
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"address_attributes" => {
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"street" => "Nieuwe Prinsengracht"
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}
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}
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}
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```
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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.
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#### Nested form for a collection of associated objects
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The form code for an association collection is pretty similar to that of a single associated object:
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```erb
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<%= form_for @person do |f| %>
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<%= f.text_field :name %>
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<%= f.fields_for :projects do |pf| %>
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<%= pf.text_field :name %>
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<% end %>
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<% end %>
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```
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Which generates:
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```html
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<form action="/people" class="new_person" id="new_person" method="post">
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<input id="person_name" name="person[name]" type="text" />
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<input id="person_projects_attributes_0_name" name="person[projects_attributes][0][name]" type="text" />
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<input id="person_projects_attributes_1_name" name="person[projects_attributes][1][name]" type="text" />
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</form>
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```
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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:
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```ruby
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{
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"person" => {
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"name" => "Eloy Duran",
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"projects_attributes" => {
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"0" => { "name" => "Project 1" },
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"1" => { "name" => "Project 2" }
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}
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}
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}
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```
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You can basically see the `projects_attributes` hash as an array of attribute hashes, one for each model instance.
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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.
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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.
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