sql - Rails - has_one 关系 : scopes for associated and non-associated objects

Question

我有这样的关系:用户可以拥有零只或一只狗，但狗必须属于某人。

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

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

我想定义以下范围:

User.with_a_dog
User.without_a_dog

对于第一种情况，我可以这样做，因为在 rails 中默认情况下连接是 INNER JOIN :

scope :with_a_dog, :joins(:dog)

1/第一个范围的解决方案是否足够好？

2/你会为第二个做什么？

3/(有点相关)有没有更好的方法来做到这一点？ :

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

谢谢你的帮助!

Answer 1

只是想添加这个以防有人觉得它有用:

用户名

class User < ActiveRecord::Base
  has_one :dog

  # To get records with a dog we just need to do a join.
  # AR will do an inner join, so only return records with dogs
  scope :with_a_dog, -> { joins(:dog) }

  # To get records without a dog, we can do a left outer join, and then only
  # select records without a dog (the dog id will be blank).
  # The merge with the Dog query ensures that the id column is correctly
  # scoped to the dogs table
  scope :without_a_dog, -> {
    includes(:dog).merge( Dog.where(:id => nil) )
  }
end

狗

class Dog < ActiveRecord::Base
  belongs_to :user
end