associations - Ruby-on-Rails: Multiple has_many :through possible?

Question

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?

Answer 1

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)