Understanding :source option of has_one/has_many through of Rails

Question

Please help me in understanding the :source option of has_one/has_many :through association. The Rails API explanation makes very little sense to me.

"Specifies the source association name used by has_many :through => :queries. Only use it if the name cannot be inferred from the association. has_many :subscribers, :through => :subscriptions will look for either :subscribers or :subscriber on Subscription, unless a :source is given. "

Answer 1

Sometimes, you want to use different names for different associations. If the name you want to use for an association on the model isn't the same as the assocation on the :through model, you can use :source to specify it.

I don't think the above paragraph is much clearer than the one in the docs, so here's an example. Let's assume we have three models, Pet, Dog and Dog::Breed.

class Pet < ActiveRecord::Base
  has_many :dogs
end

class Dog < ActiveRecord::Base
  belongs_to :pet
  has_many :breeds
end

class Dog::Breed < ActiveRecord::Base
  belongs_to :dog
end

In this case, we've chosen to namespace the Dog::Breed, because we want to access Dog.find(123).breeds as a nice and convenient association.

Now, if we now want to create a has_many :dog_breeds, :through => :dogs association on Pet, we suddenly have a problem. Rails won't be able to find a :dog_breeds association on Dog, so Rails can't possibly know which Dog association you want to use. Enter :source:

class Pet < ActiveRecord::Base
  has_many :dogs
  has_many :dog_breeds, :through => :dogs, :source => :breeds
end

With :source, we're telling Rails to look for an association called :breeds on the Dog model (as that's the model used for :dogs), and use that.