Ruby on Rails Nested Attributes

Introduction

Nested Attributes is a feature that allows you to save attributes of a record through its associated parent. In this example we’ll consider the following scenario:

We’re making an online store with lots of products. Each Product can have zero or more Variants. Variants are exactly what they sound like; they represent a variation of the same product, but in different color, for example. Both have a name and price. Each product will also be associated with one Image record, containing a url, alt and a caption.

Later in the tutorial we’ll improve these models:

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class Product < ActiveRecord::Base
  has_many :variants
  has_one :image
  # Attributes: name:string, price:float
end

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class Variant < ActiveRecord::Base
  belongs_to :product
  # Attributes: name:string, price:float
end

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class Image < ActiveRecord::Base
  belongs_to :product
  # Attributes: url:string, alt:string caption:string
end

One-to-one association

The simplest example of Nested Attributes is with a one-to-one association. To add Nested Attributes support to the product model all you need to do is add the following line:

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class Product < ActiveRecord::Base
  has_many :variants
  accepts_nested_attributes_for :image
end

What does this do, exactly? It would proxy the saved attributes from the Product model to the Image model. In the Product form you’ll need to add the additional fields for the image association. You can do it by using the fields_for helper.

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= form_for @product do |f|

  // Product attributes
  .form-group
    = f.label :name
    = f.text_field :name  
  .form-group
    = f.label :price
    = f.text_field :price

  // Image attributes
  = f.fields_for :image do |f|
    = f.label :url
    = f.text_field :url

    = f.label :alt
    = f.text_field :alt

    = f.label :caption
    = f.text_field :caption

  = f.submit

Now, the only remaining part is to modify the controller to accept those new attributes. The entire idea behind Nested Attributes is that you won't have to add additional code in the controller to handle this input and the association, but you do need to allow those attributes to reach the model, which is what Strong Parameters would prevent by default. So, you’ll need to add the following to the product_params method in the ProductsController.

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def product_params
  params.require(:product).permit(
    :name, :price,
    image_attributes: [ :id, :url, :alt, :caption ]
  )
end

And voila! Now you can edit the Image association of the Product model inline from the same form. Now let's look at how we will build the same behavior with a many-to-many relationship.

Many-to-many association

Product variants are quite simple (just two fields), so there’s really no point in creating a separate page for editing them. Instead, we would like to edit them inline from the same product form, along with the product attributes. And since each product can have many variants that means that we’ll have to handle more than one item. We’ll also need to add new variants and delete old ones. Let's address the problems one by one.

Displaying multiple associations

The fields_for method yields a block for each associated record, so we don't need to change anything--but because we will need to reuse this form (for the purpose of automatically adding new fields through JavaScript) we’ll need to move it into a separate file. We’re going to create a new partial, called _variant_fields.slim containing just the variant fields, like this:

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= f.label :name
= f.text_field :name

= f.label :price
= f.text_field :price

And back in the product form, to render the fields, we’ll just take advantage of the fact that fields_for yields a block for each association and we’ll pass the form helper object to the partial.

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= f.fields_for :variants do |f|
  = render 'variant_fields', f: f

Adding new associations

In order to add new associations we’ll need to create some JavaScript that adds new fields. What I like to do is have a link that, when clicked, will add a new tuple of fields. Something like this:

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= link_to_add_fields 'Add Product Variant', f, :variants

This is a useful helper method I wrote that will create a link with the data-form-prepend attribute containing the entire contents of the _variant_fields.slim partial. The idea here is that when you click on it you’ll use some simple reusable JavaScript to append those fields to the end of the form.

The actual helper looks quite complex and messy but bear with me--I promise that it’s just as simple as most of the code. It just handles the arguments and the key logic rests in the last seven lines. You can place this code in your application_helper.rb.

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def link_to_add_fields(name = nil, f = nil, association = nil, options = nil, html_options = nil, &block)
  # If a block is provided there is no name attribute and the arguments are
  # shifted with one position to the left. This re-assigns those values.
  f, association, options, html_options = name, f, association, options if block_given?

  options = {} if options.nil?
  html_options = {} if html_options.nil?

  if options.include? :locals
    locals = options[:locals]
  else
    locals = { }
  end

  if options.include? :partial
    partial = options[:partial]
  else
    partial = association.to_s.singularize + '_fields'
  end

  # Render the form fields from a file with the association name provided
  new_object = f.object.class.reflect_on_association(association).klass.new
  fields = f.fields_for(association, new_object, child_index: 'new_record') do |builder|
    render(partial, locals.merge!( f: builder))
  end

  # The rendered fields are sent with the link within the data-form-prepend attr
  html_options['data-form-prepend'] = raw CGI::escapeHTML( fields )
  html_options['href'] = '#'

  content_tag(:a, name, html_options, &block)
end

On the JavaScript side I use a jQuery to find every element with the name attribute set to new_record and replace it with a timestamp. This solves a problem when adding more than one new record; both will have the same id (new_record).

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$("[data-form-prepend]").click(function(e) {
  var obj = $($(this).attr("data-form-prepend"));
  obj.find("input, select, textarea").each(function() {
    $(this).attr("name", function() {
      return $(this)
        .attr("name")
        .replace("new_record", new Date().getTime());
    });
  });
  obj.insertBefore(this);
  return false;
});

Deleting associations

Fortunately the accepts_nested_attributes_for has some neat features for deleting associations. If we pass the allow_destroy: true argument to accepts_nested_attributes_for, it will destroy any members from the attributes which contain a _destroy key.

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accepts_nested_attributes_for :variants, allow_destroy: true

In the view this could be implemented with a simple checkbox. So I added one to my _variant_fields.slim:

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= f.check_box :_destroy
= f.label :delete

Modifications to the Model and Controller

Again, as before the additions to the ProductsController are just in the product_params method, which now should also include the variants_attributes.

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def product_params
  params.require(:product).permit(
    :name, :price,
    image_attributes: [ :id, :url, :alt, :caption ],
    variants_attributes: [ :id, :name, :price, :_destroy ]
  )
end

The Product model just has the following addition to enable Nested Attributes for the variants association:

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accepts_nested_attributes_for :variants, reject_if: :all_blank, allow_destroy: true

Notice the reject_if :all_blank option. It means that any record In which attributes are all blank (excluding the value of _destroy) will be rejected. reject_if also supports passing it a Proc, which can be used for some additional validation, and it also checks whether to reject/include the association.

There are two other useful options. The first is the limit option which specifies the maximum number of records that will be processed. The second option is update_only which applies to one-to-one associations and has a rather interesting behavior. If it’s set to true, it will only update the attributes of the associated record. If set to false upon change, it won't touch the old record but will create a new one with the new attributes. By default it is false and will create a new record unless the record includes an id attribute, which is exactly the reason why we included the id attribute in the product_params for the one-to-one image association. An alternative solution would have been to define the nested attributes like this:

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accepts_nested_attributes_for :image, update_only: true

You can read more about Nested Attributes in the Ruby on Rails documentation where each of the configuration options is demonstrated and well documented.

About the author

Itay Grudev is a student currently pursuing a degree in Computer Science and Physics at the University of Aberdeen, United Kingdom.

Itay is mostly interested in Linux, Security, Electronics and Amateur Radio. He loves Open Source and Free Software. He is a talented developer and one of those people spending time to change your i++ to ++i, crazy about efficiency and beautiful code. His favorite technologies are C++, Qt and Ruby on Rails.