Rails 4通过has_many的计数顺序

Question

In my case, users are related to other users through friendships. I want to order my users by the number of friends/friendships they have.

class User < ActiveRecord::Base
    has_many :friendships
    has_many :inverse_friendships, :class_name => "Friendship", :foreign_key => "friend_id"
end


class Friendship < ActiveRecord::Base
    belongs_to :user
    belongs_to :friend, :class_name => "User"
end

NB In my case I am using postgres

I am currently using

a = User.all.collect {|x| [x.id, x.friends.length]}; 
a = a.sort {|x,y| y[-1] <=> x[-1]}

Where x.friends is a method which returns all friends via Friendship.where('friend_id = ? OR user_id = ?', self, self) . I believe this is suboptimal as it makes a request per user.

Answer 1

I am not sure that my request will work properly, because I can't test on on a real DB now. But you should do it like this:

User.select(users.*, SUM(friendships.id)).joins("INNER JOIN users as friendships ON users.id = friendships.friend_id").group(users.id).order("SUM(friendships.id)")

User.select(users.*, SUM(inverse_friendships.id)).joins("INNER JOIN users as inverse_friendships ON users.friend_id = inverse_friendships.id").group(users.id).order("SUM(inverse_friendships.id)")

It will work only with PostgreSQL 9.1 and higher.