:has_many :through associations wtihout convention names?

create_table "friendships", :force => true do |t|
    t.integer  "user_id"
    t.integer  "friend_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "likes", :force => true do |t|
    t.string   "name"
    t.integer  "user_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "users", :force => true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

And these are the models

class User < ActiveRecord::Base

    has_many :friendships
    has_many :friends, :through => :friendships
    has_many :likes
    has_many :friends_likes, :through => :friendships, :source => :likes

end

class Friendship < ActiveRecord::Base

    belongs_to :user
    belongs_to :friend, :class_name => "User", :fore开发者_如何学Pythonign_key => "friend_id"
    has_many :likes, :foreign_key => :user_id, 
end

class Like < ActiveRecord::Base

    belongs_to :user
    belongs_to :friendship

end

I'm trying to get "likes of friends" but i can't.

"User.find(1).friends_likes" gives that sql query

SELECT "likes".* FROM "likes" INNER JOIN "friendships" ON "likes".user_id = "friendships".id WHERE (("friendships".user_id = 1))

but i think that has to be "INNER JOIN "friendships" ON "likes".user_id = "friendships".friend_id"

how can i do that? thanks

The simplest solution is probably to add an instance method friends_likes on the User model that constructs the correct SQL:

  def likes_of_friends
    friends.includes(:likes).map(&:likes)
  end

The .includes(:likes) is for performance, to avoid an N+1 query situation.

Then your User.find(1).friends_likes will produce the following queries, assuming user 1 has friends with ids 2 and 3:

  User Load (0.1ms)  SELECT `users`.* FROM `users` LIMIT 1
  User Load (0.4ms)  SELECT `users`.* FROM `users` INNER JOIN `friendships` ON `users`.id = `friendships`.friend_id WHERE ((`friendships`.user_id = 1))
  Like Load (0.2ms)  SELECT `likes`.* FROM `likes` WHERE (`likes`.user_id IN (2,3))

If you really need everything in one query, you could write straight-up SQL:

Like.find_by_sql("select likes.* from
  likes
    inner join users as friends
      on friends.id = likes.user_id
    inner join friendships
      on friendships.friend_id = friends.id
  where friendships.users_id = 1;
")

The reason it isn't more straightforward is because one User "owns" the friendship - it's one-way, and there doesn't seem to be a way to get the Friendships associated with the "Friend" (specified by friend_id on the friendships table).

So, adding that opposite direction will help (weird naming aside):

class Friendships
  # ...
  has_many :befriendedships, :class_name => "Friendship", :foreign_key => "friend_id"
end

Then, you can query for the thing you're looking for a bit more simply:

Like.joins(:user => :befriendedships).where(["friendships.user_id = ?", 1])

That'll generate essentially the same SQL as the find_by_sql example.