Nested Forms in Rails

3 minute read

Nested Forms in Rails allow you to handle multiple models, including creating and saving attributes of different models, from a single form. A functional nested form requires modification to the form view, relevant form models, as well as the controller for the primary form object. This walkthrough will provide code snippets for each element of our example MVC (Model, View, Controller) that is necessary to create a working nested form.

In order to take advantage of magical Rails Form Builder Helpers, the intended associations between your application models must be set up by defining the associations in your models and in respective database table migrations.

For our example, we will create a dream journal with a form to record the content of a dream and tag relevant dream themes. The nested form will focus on the dream and theme models.


Our model relationships are specified as each dream has_many themes and a theme belongs_to a dream.

# app/models/dream.rb

class Dream
  has_many :themes
  accepts_nested_attributes_for :themes


# app/models/theme.rb

class Theme
  belongs_to :dream


accepts_nested_attributes_for :themes

This class method is essential to our nested form and required for mass assignment. The accepts_nested_attributes_for method will allow us to save theme attributes through the dream model within our ​form_for(@dream)​. As a result, an attribute writer for themes is created. The attribute writer in our example will be: themes_attributes=(attributes)


Within the new action we are instantiating a new instance of a dream; this will allow us to use the form_for macro, essentially telling the form what model we are building the form for. Next we will use the collection.build method to build a new collection of themes linked to the dream instance. We gain the collection.build method when we use the has_many macro in the dream model.

If we were working with the belongs_to macro, the association_build method would replace the collection.build method.

Lastly, we must update the strong params to include the themes_attributes hash. This addition will permit the themes_attributes hash to be used in mass assignment within the create action.

# app/controllers/dreams_controller.rb

class DreamsController < ApplicationController
  def new
   @dream = Dream.new
   # This creates an empty theme collection

 def create
  @dream = Dream.create(dream_params)
   if @dream.save
    redirect_to dream_path(@dream)
    render new


  def dream_params
    params.require(:dream).permit(:title, :content, theme_id, themes_attributes: [:name])



The form we intend to build should store the user submitted data in the following params structure:

params = {
  dream => {
    :title => 'Space Journey',
    :content => 'Fusce fermentum. Curabitur suscipit suscipit tellus. Suspendisse potenti. Vestibulum ullamcorper mauris   
		at ligula.',
    :themes_attributes => [
      {:name => 'space'},
      {:name => 'aliens'}


Instantiating a new dream instance in the DreamController new action allows us to take advantage of the form_form helper. The dream instance (​@dream​) passed to the form builder informs the form builder object of the model we are creating the form for.

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

# this form is for a dream object
<%= form_form(@dream) do |f| %>

  <%= f.label :title %>
  <%= f.text_field :title %>
  <%= f.label :content %>
  <%= f.text_field :content %>
  Select related themes:
  <%= f.collection_check_boxes :theme_id, Theme.all, :id, :name %>
  Or, add a new theme:
  <%= f.fields_for :themes do |theme|
    <%= theme.label :name %>
    <%= theme.text_field :name %>
  <% end %>
  <%= f.submit %>
<%= end %>

The fields_for will nest the value of the newly added theme to the dream_params. The HTML form input tag will resemble our desired params structure and render as: <input type=“text” name=“dream[themes_attributes][0][name]” id=“dream_themes_attributes_0_name”>.

Lastly, we must update the dream model to prevent the creation of blank themes in the nested attribute.

# /models/dream.rb

class Dream
  has_many :themes
  accepts_nested_attributes_for :themes, reject_if: :all_blank