add_multiple_associated_validation_callbacks

# File activerecord/lib/active_record/associations.rb, line 1346
        def add_multiple_associated_validation_callbacks(association_name)
          method_name = "validate_associated_records_for_#{association_name}".to_sym
          ivar = "@#{association_name}"

          define_method(method_name) do
            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)

            if association.respond_to?(:loaded?)
              if new_record?
                association
              elsif association.loaded?
                association.select { |record| record.new_record? }
              else
                association.target.select { |record| record.new_record? }
              end.each do |record|
                errors.add association_name unless record.valid?
              end
            end
          end

          validate method_name
        end