has_many :through a has_and_belongs_to_many association

activerecord, ruby-on-rails

Solution

My first version of the answer was incorrect, but this one works perfectly. I made a couple of typos the first time (the hazard of not actually creating an app to test) but this time I verified. And a plugin is needed, but this is easy. first, install the plugin:

script/plugin install git://github.com/ianwhite/nested_has_many_through.git

This installs Ian White's workaround, and it works seamlessly. Now the models, copied directly from the test app I setup to get this working:

class FoodItem < ActiveRecord::Base
  has_many :food_category_items
  has_many :food_categories, :through => :food_category_items
  has_many :places, :through => :food_categories
end

class FoodCategory < ActiveRecord::Base
  has_many :food_category_items
  has_many :food_items, :through => :food_category_items
  belongs_to :place
end

class FoodCategoryItem < ActiveRecord::Base
  belongs_to :food_item
  belongs_to :food_category
end

class Place < ActiveRecord::Base
  has_many :food_categories
  has_many :food_category_items, :through => :food_categories
  has_many :food_items, :through => :food_category_items
end

Now "far" associations work just as well. `place_instance.food_items` and `food_item.places` both work flawlessly, as well as the simpler associations involved. Just for reference, here's my schema to show where all the foreign keys go:

create_table "food_categories", :force => true do |t|
  t.string   "name"
  t.integer  "place_id"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "food_category_items", :force => true do |t|
  t.string   "name"
  t.integer  "food_item_id"
  t.integer  "food_category_id"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "food_items", :force => true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "places", :force => true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

Hope this helps!

UPDATE: This question has come up a few times recently. I wrote an article, nesting your has_many :through relationships, to explain in detail. It even has an accompanying example application on GitHub to download and play around with.

Problem

Original source