ActiveRecord关系验证，尝试使用“计数”时出错

Question

I have an Order class, and a Pack class, both using ActiveRecord. The Order contains Packs. In my validation of Order, I am testing for the existence of a relationship to one or more Packs. See the following code:

class Order < ActiveRecord::Base
  belongs_to :user
  has_many :order_packs
  has_many :packs, :through => :order_packs
  validate :my_custom_validation

  def my_custom_validation
    errors.add(:packs, "Your order was empty.") if packs.count < 1
  end
end

Seems simple enough, but it doesn't work; packs.count is always zero. So I changed the validation to this code:

def my_custom_validation
  errors.add(:packs, "packs is: #{packs}")
  errors.add(:packs, "packs.count is: #{packs.count}")
  errors.add(:packs, "packs.any? is: #{packs.any?}")
end

just to see what the deal was, and got this interesting output:

Can anyone tell me why count is zero?

Answer 1

Try using .length instead of .count .

When you use count you're actually performing a database query. And because you're doing it on validation, before anything has been saved to the database, you're always getting zero. length on the other hand works on the object level and doesn't hit the database at all. So it should work for you.

Answer 2

length and size, both are same, and count performs SQL COUNT query,

You should read count vs length vs size