Rails sort entries based on attributes of related model

I have a strange scenario: I would like to order all posts P by the time at which P's creator and the current user became friends.

The result would be a list of posts with newer friends' posts at the top and older friends' posts at the bottom. We can assume all users are friends of all other users, but each friendship has varying creation times.

I have Post, User, and Friendship models. Posts belong to users and users have many friendships.

class User < ActiveRecord::Base
  has_many :friendships
  has_many :friends, :through => :friendships
  has_many :inverse_friendships, :class_name => "Friendship", :foreign_key => "friend_id"
  has_many :inverse_friends, :through => :inverse_friendships, :source => :user

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

class Post < ActiveRecord::Base
  belongs_to :user

What's the best way to do this? Thanks!


You can order on the associated table by doing a nested join, like so:

@posts = Post.joins(:user => :friendships).order("friendships.friended_at")

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