add_single_associated_validation_callbacks

# File activerecord/lib/active_record/associations.rb, line 1334
        def add_single_associated_validation_callbacks(association_name)
          method_name = "validate_associated_records_for_#{association_name}".to_sym
          define_method(method_name) do
            association = instance_variable_get("@#{association_name}")
            if !association.nil?
              errors.add association_name unless association.target.nil? || association.valid?
            end
          end

          validate method_name
        end