Posted over 9 years ago. Visible to the public. Repeats.

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.

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Owner of this card:

Henning Koch
Last edit:
about 6 years ago
by Henning Koch
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Henning Koch to makandra dev
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