Associations on collections - ActiveRecord

foo.rb

# Currently, you have to roll your own associations if you want to chain scopes through collections.
# I'm interested to see if there is an existing (generalised) way to do this with AR.
# If not, feedback on the value and interface (method chaining?) for collection associations would be appreciated.

# gem install activerecord
# gem install sqlite3

require 'active_record'

ActiveRecord::Base.establish_connection(:adapter => 'sqlite3', :database => ':memory:')
ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :posts do |table|
  end

  create_table :comments do |table|
    table.column :post_id, :integer
  end
end

class Post < ActiveRecord::Base
  has_many :comments

  def self.comments
    Comment.where(:post_id => scoped.pluck(primary_key).uniq)
  end
end

class Comment < ActiveRecord::Base
  belongs_to :post

  def self.posts
    Post.where(:id => scoped.pluck(:post_id).uniq)
  end
end

post = Post.create!
10.times { post.comments.create! }
puts Post.limit(10).comments.posts.comments.posts.to_sql