Traverse an ActiveRecord relation along an association

The Edge Rider gem Archive 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.

Henning Koch almost 9 years ago
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