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.
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
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)))
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)))
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.
Am I doing something wrong, or is this simply not possible?
Edit:
Rails 3.1 supports nested associations. E.g:
has_many :tasks
has_many :assigments, :through => :tasks
has_many :users, :through => :assignments
There is no need for the solution given below. Refer to this screencast for more details.
Original Answer
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
You have three approaches to solving this issue.
1) Write an association extension
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
Now you can get the friends comments as follows:
user.friends.comments
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
Now you can get the friends comments as follows:
user.friends_comments
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
Now you can get the friends comments as follows:
user.friends_comments
All methods cache the results. If you want to reload the results do the following:
user.friends_comments(true)
user.friends.comments(true)
OR better still:
user.friends_comments(:reload)
user.friends.comments(:reload)