Ruby-on-Rails:多个 has_many :through 可能吗?
是否可以在 Rails 中拥有多个相互传递的 has_many :through
关系?我收到了这样做的建议,作为我发布的另一个问题的解决方案,但一直无法让它发挥作用.
Is it possible to have multiple has_many :through
relationships that pass through each other in Rails? I received the suggestion to do so as a solution for another question I posted, but have been unable to get it to work.
朋友是通过连接表的循环关联.目标是为 friends_comments
创建一个 has_many :through
,这样我就可以使用 User
并执行类似 user.friends_comments 在一个查询中获取他的朋友的所有评论.
Friends are a cyclic association through a join table. The goal is to create a has_many :through
for friends_comments
, so I can take a User
and do something like user.friends_comments
to get all comments made by his friends in a single query.
class User
has_many :friendships
has_many :friends,
:through => :friendships,
:conditions => "status = #{Friendship::FULL}"
has_many :comments
has_many :friends_comments, :through => :friends, :source => :comments
end
class Friendship < ActiveRecord::Base
belongs_to :user
belongs_to :friend, :class_name => "User", :foreign_key => "friend_id"
end
这看起来不错,也有道理,但对我不起作用.这是我在尝试访问用户的朋友评论时在相关部分遇到的错误:错误:列 users.user_id 不存在
: SELECT "comments".* FROM "comments" INNER JOIN "users" ON "comments".user_id = "users".id WHERE (("users".user_id = 1) AND ((status = 2)))代码>
This looks great, and makes sense, but isn't working for me. This is the error I'm getting in relevant part when I try to access a user's friends_comments:ERROR: column users.user_id does not exist
: SELECT "comments".* FROM "comments" INNER JOIN "users" ON "comments".user_id = "users".id WHERE (("users".user_id = 1) AND ((status = 2)))
当我刚刚输入 user.friends 时,它起作用了,这是它执行的查询:: SELECT "users".* FROM "users" INNER JOIN "friendships" ON "users".id = "friendships".friend_id WHERE (("friendships".user_id = 1) AND ((status = 2)))
When I just enter user.friends, which works, this is the query it executes:: SELECT "users".* FROM "users" INNER JOIN "friendships" ON "users".id = "friendships".friend_id WHERE (("friendships".user_id = 1) AND ((status = 2)))
所以看起来它完全忘记了原来的has_many
,通过友谊关系,然后不恰当地尝试使用User类作为连接表.
So it seems like it's entirely forgetting about the original has_many
through friendship relationship, and then is inappropriately trying to use the User class as a join table.
我做错了什么,还是根本不可能?
Am I doing something wrong, or is this simply not possible?
Rails 3.1 支持嵌套关联.例如:
Rails 3.1 supports nested associations. E.g:
has_many :tasks
has_many :assigments, :through => :tasks
has_many :users, :through => :assignments
不需要下面给出的解决方案.有关详细信息,请参阅此截屏视频.
There is no need for the solution given below. Refer to this screencast for more details.
原答案
您正在传递一个 has_many :through
关联作为另一个 has_many :through
的源协会.我不认为它会起作用.
You are passing a has_many :through
association as a source for another has_many :through
association. I don't think it will work.
has_many :friends,
:through => :friendships,
:conditions => "status = #{Friendship::FULL}"
has_many :friends_comments, :through => :friends, :source => :comments
您可以通过三种方法解决此问题.
You have three approaches to solving this issue.
1) 编写关联扩展
has_many :friends,
:through => :friendships,
:conditions => "status = #{Friendship::FULL}" do
def comments(reload=false)
@comments = nil if reload
@comments ||=Comment.find_all_by_user_id(map(&:id))
end
end
现在你可以得到朋友的评论如下:
Now you can get the friends comments as follows:
user.friends.comments
2) 向 User
类添加方法.
2) Add a method to the User
class.
def friends_comments(reload=false)
@friends_comments = nil if reload
@friends_comments ||=Comment.find_all_by_user_id(self.friend_ids)
end
现在你可以得到朋友的评论如下:
Now you can get the friends comments as follows:
user.friends_comments
3) 如果你想让它更有效率,那么:
3) If you want this to be even more efficient then:
def friends_comments(reload=false)
@friends_comments = nil if reload
@friends_comments ||=Comment.all(
:joins => "JOIN (SELECT friend_id AS user_id
FROM friendships
WHERE user_id = #{self.id}
) AS friends ON comments.user_id = friends.user_id")
end
现在你可以得到朋友的评论如下:
Now you can get the friends comments as follows:
user.friends_comments
所有方法都会缓存结果.如果要重新加载结果,请执行以下操作:
All methods cache the results. If you want to reload the results do the following:
user.friends_comments(true)
user.friends.comments(true)
或者更好:
user.friends_comments(:reload)
user.friends.comments(:reload)