我在Ruby on Rails中制作了一个社交网络应用程序,当我在Micropost模型中使用where方法内部和类方法时,我看到了一个有趣的行为,工作没有在它前面指定的类(像“Micropost.where()”)Rails,其中类方法中的功能vs实例方法中的功能
class Micropost < ActiveRecord::Base
attr_accessible :content
belongs_to :user
validates :user_id, presence: true
validates :content, presence: true, length: { maximum: 255 }
default_scope order: "microposts.created_at DESC"
def self.from_users_followed_by(user)
followed_user_ids = "SELECT followed_id FROM relationships
WHERE follower_id = :user_id"
where("user_id IN (#{followed_user_ids}) OR user_id = :user_id",
user_id: user.id)
end
end
但是就像下面,它需要知道该模型的名字时,我使用它在一个实例方法。
class User < ActiveRecord::Base
...
...
def friends()
sql_direct_friends = "SELECT friend_id FROM friendships
WHERE approved = :true_value AND user_id = :user_id"
sql_indirect_friends = "SELECT user_id FROM friendships
WHERE approved = :true_value AND friend_id = :user_id"
User.where("id IN (#{sql_direct_friends}) OR id IN (#{sql_indirect_friends})", user_id: id, true_value: true)
end
end
然后,如果我使用“其中”,而不是“User.where”然后我会得到这样的错误:
NoMethodError: undefined method `where' for #<User:0x00000004b908f8>
这是为什么发生? friends()方法中的where方法是否认为我将它用作当前对象(self.friends())上的实例方法?