Ruby on Rails before_destroy导致NameError

Question

我试图在模型上实现before_destroy.虽然我遵循这个before_destroy示例,但我收到一个NameError或者最终得到的东西不能执行before_destroy.我的语法有什么问题？

class Person < ActiveRecord::Base

  has_many :book_loans, :order => 'return_date desc'
  has_many :books, :through => :book_loans

  before_destroy
    errors.add_to_base "Cannot delete borrower with loans" unless
                                                           book_loans.count == 0
end

不编译

  before_destroy
    errors.add_to_base "Cannot delete borrower with loans" unless
                                                           book_loans.count == 0

不起作用

def before_destroy
  errors.add_to_base "Cannot delete borrower with loans" unless
                                                         book_loans.count == 0
end

Answer 1

您无法添加错误before_destroy,因为它们不会影响验证.您必须在验证发生之前添加错误,例如before_validation.

还有一些额外的回调,before_validation_on_create和before_validation_on_update.on_destroy但是,没有版本,因为save并且valid?永远不会被调用,因此当您尝试销毁某些内容时添加验证错误将毫无意义.

换句话说:您的before_destroy运行,但它不会影响任何内容,因为在销毁记录时没有检查验证错误.

这里有一个实现可以根据特定条件阻止记录保存的东西.

before_destroy :require_no_book_loans

private

def require_no_book_loans
  raise ActiveRecord::RecordInvalid unless book_loans.count == 0
end

您还可以创建自己的错误类并提高它.

仅供参考:您应该before_destroy在语法错误示例中传递一个块.

before_destroy {|r|
  r.errors.add_to_base "Foo" # ...
}