在 rails 3 中查找没有关联记录的记录 [英] Finding records with no associated records in rails 3

问题描述

class Person < ActiveRecord::Base
  has_many :pets

  scope :with_dog, join(:pets).where("pets.type = 'Dog'")
  scope :without_pets ???????????????????????????????????
end

class Pet < ActiveRecord::Base
  belongs_to :people
end

我想向 Person 模型添加一个范围，以返回没有宠物的人.有任何想法吗?我觉得这很明显，但此刻它正在逃避我.

I'd like to add a scope to the Person model that returns people who have no pets. Any ideas? I feel like this is obvious, but it's escaping me at the moment.

推荐答案

试试这个:

Person.joins('left outer join pets on persons.id=pets.person_id').
       select('persons.*,pets.id').
       where('pets.id is null')

我还没有测试过，但它应该可以工作.

I haven't tested it but it ought to work.

这个想法是我们正在执行左外连接，因此对于每个没有宠物的人来说，pets 字段将为空.您可能需要包含 :readonly =>连接中的 false，因为当 join() 传递一个字符串时，ActiveRecord 返回只读对象.

The idea is that we're performing a left outer join, so the pets fields will be null for every person that has no pets. You'll probably need to include :readonly => false in the join since ActiveRecord returns read-only objects when join() is passed a string.

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