rails/activemodel/lib/active_model/validations/associated.rb

module ActiveModel
  module Validations
    module ClassMethods
      # Validates whether the associated object or objects are all valid themselves. Works with any kind of association.
      #
      #   class Book < ActiveRecord::Base
      #     has_many :pages
      #     belongs_to :library
      #
      #     validates_associated :pages, :library
      #   end
      #
      # Warning: If, after the above definition, you then wrote:
      #
      #   class Page < ActiveRecord::Base
      #     belongs_to :book
      #
      #     validates_associated :book
      #   end
      #
      # ...this would specify a circular dependency and cause infinite recursion.
      #
      # NOTE: This validation will not fail if the association hasn't been assigned. If you want to ensure that the association
      # is both present and guaranteed to be valid, you also need to use +validates_presence_of+.
      #
      # Configuration options:
      # * <tt>:message</tt> - A custom error message (default is: "is invalid")
      # * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>)
      # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
      #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).  The
      #   method, proc or string should return or evaluate to a true or false value.
      # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine if the validation should
      #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).  The
      #   method, proc or string should return or evaluate to a true or false value.
      def validates_associated(*attr_names)
        configuration = { :message => ActiveRecord::Errors.default_error_messages[:invalid], :on => :save }
        configuration.update(attr_names.extract_options!)

        validates_each(attr_names, configuration) do |record, attr_name, value|
          record.errors.add(attr_name, configuration[:message]) unless
            (value.is_a?(Array) ? value : [value]).inject(true) { |v, r| (r.nil? || r.valid?) && v }
        end
      end
    end
  end
end