In Rails 5, we have the option index_errors: true to output the nested item index for errors like this:
class Order < ApplicationRecord
has_many :items, index_errors: true
accepts_nested_attributes_for :items
end
order = Order.create(items_attributes: [{ price: 1 }, { price: -1 }])
order.errors # #<ActiveModel::Errors... @messages={:*"items[1].price"*: ["must be greater than 0"]}>BUT IT DOESN'T WORK FOR RAILS 4, so... let's monkey patch it :)
Include to your Gemfile
gem 'active_record-nested_error_indexer'