Rails 3: Uniqueness validation for nested fields_for

Question

A have two models, \"shop\" and \"product\", linked via has_many :through.

In the shop form there are nested attributes for multiple products, and I\'m having a litt

Answer 1

To expand on Alberto's solution, the following custom validator accepts a field (attribute) to validate, and adds errors to the nested resources.

# config/initializers/nested_attributes_uniqueness_validator.rb
class NestedAttributesUniquenessValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value.map(&options[:field]).uniq.size == value.size
      record.errors[attribute] << "must be unique"
      duplicates = value - Hash[value.map{|obj| [obj[options[:field]], obj]}].values
      duplicates.each { |obj| obj.errors[options[:field]] << "has already been taken" }
    end
  end
end

# app/models/shop.rb
class Shop < ActiveRecord::Base
  validates :products, :nested_attributes_uniqueness => {:field => :name}
end