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rails plugin that allows has_many :through to go through other has_many :throughs
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SPECDOC Updated SPECDOC with edge specs Apr 30, 2008


NOTE: rails 2.3 users might want to try the experimental rails-2.3 branch


A fantastic patch/plugin has been floating around for a while:

obrie made the original ticket and Matt Westcott released the first version of the plugin, under the MIT license. Many others have contributed, see the trac ticket for details.

Here is a refactored version (I didn't write the original), suitable for edge/2.0-stable with a bunch of acceptance specs. I'm concentrating on plugin usage, once it becomes stable, and well enough speced/understood, then it's time to pester rails-core.

Why republish this on github?

  • The previous implementations are very poorly speced/tested, so it's pretty hard to refactor and understand this complicated bit of sql-fu, especially when you're aiming at a moving target (edge)

  • the lastest patches don't apply on edge

  • github - let's collab to make this better and get a patch accepted, fork away!

Help out

I'm releasing 'early and often' in the hope that people will use it and find bugs/problems. Report them at, or fork and pull request, yada yada.


Here's the original description:

This plugin makes it possible to define has_many :through relationships that
go through other has_many :through relationships, possibly through an
arbitrarily deep hierarchy. This allows associations across any number of
tables to be constructed, without having to resort to find_by_sql (which isn't
a suitable solution if you need to do eager loading through :include as well).


Get in touch if you should be on this list

Show me the money!

Here's some models from the specs:

class Author < User
  has_many :posts
  has_many :categories, :through => :posts, :uniq => true
  has_many :similar_posts, :through => :categories, :source => :posts
  has_many :similar_authors, :through => :similar_posts, :source => :author, :uniq => true
  has_many :posts_of_similar_authors, :through => :similar_authors, :source => :posts, :uniq => true
  has_many :commenters, :through => :posts, :uniq => true

class Post < ActiveRecord::Base
  belongs_to :author
  belongs_to :category
  has_many :comments
  has_many :commenters, :through => :comments, :source => :user, :uniq => true

The first two has_manys of Author are plain vanilla, the last four are what this plugin enables

# has_many through a has_many :through
has_many :similar_posts, :through => :categories, :source => :posts

# doubly nested has_many :through 
has_many :similar_authors, :through => :similar_posts, :source => :author, :uniq => true

# whoah!
has_many :posts_of_similar_authors, :through => :similar_authors, :source => :posts, :uniq => true

# has_many through a has_many :through in another model
has_many :commenters, :through => :posts, :uniq => true

What does it run on?

Currently the master branch is running on 2.1, and 2.2 stable branches

Recent changes have made master incompatible with rails 2.0, if you want to use this with rails 2.0, then use the rails-2.0 branch.

If you want to run the CI suite, then check out garlic_example.rb (The CI suite is being cooked with garlic - git://

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