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.
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!
I'm releasing 'early and often' in the hope that people will use it and find bugs/problems. Report them at ianwhite.lighthouseapp.com, 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).
Ian White (github.com/ianwhite)
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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 end class Post < ActiveRecord::Base belongs_to :author belongs_to :category has_many :comments has_many :commenters, :through => :comments, :source => :user, :uniq => true end
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
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://github.com/ianwhite/garlic)