Traverse an ActiveRecord relation along an association

The Edge Rider gem Show archive.org snapshot gives your relations a method #traverse_association which
returns a new relation by "pivoting" around a named association.

Say we have a Post model and each Post belongs to an author:

class Post < ActiveRecord::Base
  belongs_to :author
end

To turn a relation of posts into a relation of its authors:

posts = Post.where(:archived => false)
authors = posts.traverse_association(:author)

You can traverse multiple associations in a single call.
E.g. to turn a relation of posts into a relation of all posts of their authors:

posts = Post.where(:archived => false)
posts_by_same_authors = posts.traverse_association(:author, :posts)

Implementation note

The traversal is achieved internally by collecting all foreign keys in the current relation
and return a new relation with an IN(...) query (which is very efficient even for many thousand keys).
This means every association that you pivot around will trigger one SQL query.