导轨 - HAS_ONE关系：作用域关联和非关联的对象 [英] Rails - has_one relationship : scopes for associated and non-associated objects

问题描述

我有这样的关系：用户可以有零或一只狗，但狗有属于别人

I have this relationship : a user can have zero or one dog, but a dog has to belong to someone.

# dog.rb
class Dog < ActiveRecord::Base
  belongs_to :user
end

# user.rb
class User < ActiveRecord::Base
  has_one :dog
end

我要定义的范围如下：

I want to define the following scopes :

User.with_a_dog
User.without_a_dog

我能做到这一点的第一种情况，因为联接INNER默认情况下轨JOIN：

I can do this for the first case, because the joins are INNER JOIN by default in rails :

scope :with_a_dog, :joins(:dog)

1 /这是解决方案的第一个范围不够好？

1/ Is this solution for the first scope good enough?

2 /你会怎么做第二个？

2/ What would you do for the second one?

3 /（有点关系）是否有这样做的更好的办法？

3/ (Somewhat related) Is there a better way of doing this? :

# user.rb
def has_a_dog?
  !self.dog.nil?
end

感谢您的帮助！

Thanks for your help!

推荐答案

有关问题2我认为下面应该工作：

For question 2 I think the following should work:

scope :without_a_dog include(:dog), where('dogs.id is null')

如果有应该做一个LEFT JOIN这意味着那里没有狗的关系加入到用户的dogs.id栏为空。

Where include should do a left join meaning that where there's no dog relation to join to the user the dogs.id column should be null.

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