Basic Nested Forms


  1. Construct a nested params hash with data about the primary object and a belongs_to and has_many association.
  2. Use the conventional key names for associated data (association_attributes).
  3. Name form inputs correctly to create a nested params hash with belongs_to and has_many associated data.
  4. Define a conventional association writer for the primary model to properly instantiate associations based on the nested params association data.
  5. Define a custom association writer for the primary model to properly instantiate associations with custom logic (like unique by name) on the nested params association data.
  6. Use fields_for to generate the association fields.

Data model

Let's say we're writing an address book.

Each Person can have multiple addresses. Each Address has a bunch of address info fields.

Our data model looks like this:

  • Person
    • has many addresses
    • has a name (string)
  • Address
    • has one person
    • has the first line of the street address stored as street_address_1 (string)
    • has the second line of the street address stored as street_address_2 (string)
    • has a city (string)
    • has a state (string)
    • has a zipcode (string)
    • has an address_type (string)

Creating people

How do we write our Person form? We don't want to require our user to first create an Address, then create that Person. That's annoying. We want a single form for a Person containing several slots for their addresses.

Previously, we wrote setters like Song#artist_name= to find or create an Artist and connect them to the song.

That won't work here, because an address contains more than one field. In the Artist case we were just doing the name. With Address, it's "structured data". All that really means is it has multiple fields attached to it. When we build a form for it, the form will send a different key for each field in each address. This can get a bit unwieldy so we generally try to group a hash within the params hash, which makes things much neater. Spoiler alert: Rails has a way to send this across as a hash.

The complete params object for creating a Person will look like the following. Using "0" and "1" as keys can seem a bit odd, but it makes everything else work moving forward. This hash is now more versatile. You can access nested values the standard way, with params[:person][:addresses_attributes]["0"] returning all of the information about the first address at 33 West 26th St.

  :person => {
    :name => "Avi",
    :addresses_attributes => {
      "0" => {
        :street_address_1 => "33 West 26th St",
        :street_address_2 => "Apt 2B",
        :city => "New York",
        :state => "NY",
        :zipcode => "10010",
        :address_type => "Work"
      "1" => {
        :street_address_1 => "11 Broadway",
        :street_address_2 => "2nd Floor",
        :city => "New York",
        :state => "NY",
        :zipcode => "10004",
        :address_type => "Home"

Notice the addresses_attributes key. That key is similar to the artist_name key we used previously. Last time, we handled this by writing a artist_name= method. In this case, we're going to do something super similar. Instead of writing our own addresses_attributes= method, we'll let Rails take care of it for us. We're going to use accepts_nested_attributes_for and the fields_for FormHelper.

Last time, we first wrote our setter method in the model. This time let's modify our Person model to include an accepts_nested_attributes_for :addresses line.

class Person < ActiveRecord::Base
  has_many :addresses
  accepts_nested_attributes_for :addresses


Now open up rails c and run our addresses_attributes method that was created for us by accepts_nested_attributes_for.

2.2.3 :018 > new_person =
 => #<Person id: nil, name: nil, created_at: nil, updated_at: nil>

2.2.3 :019 > new_person.addresses_attributes={"0"=>{"street_address_1"=>"33 West 26", "street_address_2"=>"Floor 2", "city"=>"NYC", "state"=>"NY", "zipcode"=>"10004", "address_type"=>"work1"}, "1"=>{"street_address_1"=>"11 Broadway", "street_address_2"=>"Suite 260", "city"=>"NYC", "state"=>"NY", "zipcode"=>"10004", "address_type"=>"work2"}}
 => {"0"=>{"street_address_1"=>"33 West 26", "street_address_2"=>"Floor 2", "city"=>"NYC", "state"=>"NY", "zipcode"=>"10004", "address_type"=>"work1"}, "1"=>{"street_address_1"=>"11 Broadway", "street_address_2"=>"Suite 260", "city"=>"NYC", "state"=>"NY", "zipcode"=>"10004", "address_type"=>"work2"}}

2.2.3 :020 >
   (0.2ms)  begin transaction
  SQL (0.8ms)  INSERT INTO "people" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2016-01-14 11:57:00.393038"], ["updated_at", "2016-01-14 11:57:00.393038"]]
  SQL (0.3ms)  INSERT INTO "addresses" ("street_address_1", "street_address_2", "city", "state", "zipcode", "address_type", "person_id", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)  [["street_address_1", "33 West 26"], ["street_address_2", "Floor 2"], ["city", "NYC"], ["state", "NY"], ["zipcode", "10004"], ["address_type", "work1"], ["person_id", 3], ["created_at", "2016-01-14 11:57:00.403152"], ["updated_at", "2016-01-14 11:57:00.403152"]]
  SQL (0.1ms)  INSERT INTO "addresses" ("street_address_1", "street_address_2", "city", "state", "zipcode", "address_type", "person_id", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)  [["street_address_1", "11 Broadway"], ["street_address_2", "Suite 260"], ["city", "NYC"], ["state", "NY"], ["zipcode", "10004"], ["address_type", "work2"], ["person_id", 3], ["created_at", "2016-01-14 11:57:00.405973"], ["updated_at", "2016-01-14 11:57:00.405973"]]
   (0.6ms)  commit transaction
 => true

This is a bit hard to read, but you'll notice that we have a method called addresses_attributes=. You didn't write that; accepts_nested_attributes_for wrote that. Then when we called it created both the Person object and the two Address objects. Boom!

Now, we just need to get our form to create a params hash like that. Easy Peasy. We are going to use fields_for to make this happen.

# app/views/people/new.html.erb

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

  <%= f.fields_for :addresses do |addr| %>
    <%= addr.label :street_address_1 %>
    <%= addr.text_field :street_address_1 %><br>

    <%= addr.label :street_address_2 %>
    <%= addr.text_field :street_address_2 %><br>

    <%= addr.label :city %>
    <%= addr.text_field :city %><br>

    <%= addr.label :state %>
    <%= addr.text_field :state %><br>

    <%= addr.label :zipcode %>
    <%= addr.text_field :zipcode %><br>

    <%= addr.label :address_type %>
    <%= addr.text_field :address_type %><br>
  <% end %>

  <%= f.submit %>
<% end %>

The fields_for line gives something nice and English-y. In that block are the fields for the addresses. Love Rails.

Load up the page, and see the majestic beauty of what you and Rails have written together. What?! Nothing is there.

Creating stubs

We're asking Rails to generate fields_for each of the Person's addresses. However, when we first create a Person, they have no addresses. Just like f.text_field :name will have nothing in the text field if there is no name, f.fields_for :addresses will have no address fields if there are no addresses.

We'll take the most straightforward way out: when we create a Person in the PeopleController, we'll add two empty addresses to fill out. The final controller looks like this:

class PeopleController < ApplicationController
  def new
    @person = 'work') 'home')

  def create
    person = Person.create(person_params)
    redirect_to people_path

  def index
    @people = Person.all


  def person_params

Now, refresh the page, and you'll see two lovely address forms. Try to hit submit, and it isn't going to work. One last hurdle. We have new params keys, which means we need to modify our person_params method to accept them. Your person_params method should now look like this:

def person_params
    addresses_attributes: [

Avoiding duplicates

One situation we can't use accepts_nested_attributes_for is when we want to avoid duplicates of the row we're creating.

In our address book app, perhaps it's reasonable to have duplicate address rows. For instance, both Jerry and Tim live on 22 Elm Street, so there are two address rows for 22 Elm Street. That's fine for those purposes.

But say we have a database of songs and artists. We would want Artist rows to be unique, so that Artist.find_by(name: 'Tori Amos').songs returns what we'd expect. If we want to be able to create artists while creating songs, we'll need to use find_or_create_by in our artist_attributes= method:

# app/models/song.rb

class Song < ActiveRecord::Base
  def artist_attributes=(artist)
    self.artist = Artist.find_or_create_by(name: artist[:name])

This looks up existing artists by name. If no matching artist is found, one is created. Then we update the artist's attributes with the ones we were given. We could also choose to do something else if we didn't want to allow bulk assigning of an artist's information through a song.

Note that accepts_nested_attributes_for and setter methods (e.g., artist_attributes=) aren't necessarily mutually exclusive. It's important to evaluate the needs of your specific use case and choose the approach that makes the most sense. Keep in mind, too, that setter methods are useful for more than just avoiding duplicates –– that's just one domain where they come in handy.

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