为什么rails模型中关联和回调的相对位置会影响结果

Question

我有一个模型 Indent，它有很多 indent_items 并且通过 indent_items 有很多发货。

class Indent < ActiveRecord::Base

  before_destroy :check_for_shipments # WORKS HERE

  has_many :indent_items, inverse_of: :indent, :dependent => :destroy
  has_many :shipments, :through => :indent_items

  # before_destroy :check_for_shipments # DOESN"T WORK HERE

  private
  def check_for_shipments
   # Should not be allowed to delete if there are any shipments.
   if self.shipments.count > 0
     errors.add(:base, "Cannot delete indent because shipments are there.")
     return false
   end    
  end

end

我猜这可能是因为如果在关联之后提到回调，所有缩进项都被标记为删除，并且发货计数检查总是返回零。

但这不应该发生。或者我可能在这里遗漏了一些东西。我不知道。

我正在使用 Rails 3.2.8。

Answer 1

这是 rails 的期望行为，如果您希望在货物销毁之前执行 before_destroy 回调（因为 dependent: :destroy 选项），那么您必须在 before_destroy 回调上使用 prepend 选项。

例如

class Indent < ActiveRecord::Base

  has_many :indent_items, inverse_of: :indent, :dependent => :destroy
  has_many :shipments, :through => :indent_items

  before_destroy :check_for_shipments, prepend: true #this will work

  private

  def check_for_shipments
   # Should not be allowed to delete if there are any shipments.
   if self.shipments.count > 0
     errors.add(:base, "Cannot delete indent because shipments are there.")
     return false
   end    
  end

end

如需进一步参考，您可以阅读from here。