This blog is part of our Rails 5 series.

We use accepts_nested_attributes_for when we want a single form to cater to multiple models. By using this we can easily provide attributes for associated models.

In Rails 4.x, if a validation fails for one or more of the associated models, then it is not possible to figure out from error message, which of the associated model object is the error related to.

class Product < ApplicationRecord
  has_many :variants
  accepts_nested_attributes_for :variants
end

class Variant < ApplicationRecord
  validates :display_name, :price, presence: true
end

>> product = Product.new(name: 'Table')
>> variant1 = Variant.new(price: 10)
>> variant2 = Variant.new(display_name: 'Brown')
>> product.variants = [variant1, variant2]
>> product.save
=> false

>> product.error.messages
=> {:"variants.display_name"=>["can't be blank"], :"variants.price"=>["can't be blank"]}

In the example above we can see that if this error message is sent as JSON API, we cannot find out which variant save failed because of which attribute.

This works well when we render forms using Active Record models, as errors are available on individual instances. But, the issue arises with an API call, where we don’t have access to these instances.

Rails 5 allows indexing of errors on nested attributes

In Rails 5, we can add an index to errors on nested models.

We can add the option index_errors: true to has_many association to enable this behavior on individual association.

class Product < ApplicationRecord
  has_many :variants, index_errors: true
  accepts_nested_attributes_for :variants
end

class Variant < ApplicationRecord
  validates :display_name, :price, presence: true
end

>> product = Product.new(name: 'Table')
>> variant1 = Variant.new(price: 10)
>> variant2 = Variant.new(display_name: 'Brown')
>> product.variants = [variant1, variant2]
>> product.save
=> false

>> product.error.messages
=> {:"variants[0].display_name"=>["can't be blank"], :"variants[1].price"=>["can't be blank"]}

Using global configuration

In order to make this change global, we can set configuration config.active_record.index_nested_attribute_errors = true which is false by default.

config.active_record.index_nested_attribute_errors = true

class Product < ApplicationRecord
  has_many :variants
  accepts_nested_attributes_for :variants
end

class Variant < ApplicationRecord
  validates :display_name, :price, presence: true
end

This will work exactly same as an example with has_many :variants, index_errors: true in Product.