Confused about class methods on association proxies in AR

ar-class-methods.rb

class Category < ActiveRecord::Base
  belongs_to :sock
  def self.foo
    relation.build(:name => "bar")
  end
end

class Sock < ActiveRecord::Base
  has_many :categories
end

# Here is what I expect:
Category.foo # => #<Category id: nil name: "bar" sock_id: nil>
Sock.find(123).categories.foo # => #<Category id: nil name: "bar" sock_id: 123>

# What I actually get is:
Sock.find(123).categories.foo # => #<Category id: nil name: "bar" sock_id: nil>

# In other words, all methods in a class method should behave as if
# they are being called against the relation representing all records
# when called from the class itself, and as if being called against
# the relation representing the association when called against an
# association proxy.

# Instead, while the class method apparently gets tacked onto the
# association proxy, it behaves as if it is being called on the class,
# not on a specific association proxy.

# What am I missing here?

# And don't say the answer is to change Sock to this:
class Sock < ActiveRecord::Base
  module CategoriesAssociation
    def foo
      relation.build(:name => "bar")
    end
  end

  has_many :categories, :extend => CategoriesAssociation
end

# The whole point here is that all categories associations should get
# this method transparently, and it should Just Work.

So it turns out the answer is to use scoped.

Can you update with it working with scoped?

This should "just work". The method_missing in CollectionProxy basically does scoped { klass.send(method_name, ...) }.

I think maybe the problem is that you are calling relation. IIRC, relation is not public and shouldn't be called by users. Why don't you just define you method as:

  def self.foo
    build(:name => "bar")
  end

?

It does not Just Work. The class method doesn't have access to association proxy methods like #build; that just raises a no method error.

Here's a writeup of the solution: http://avdi.org/devblog/2012/01/19/activerecord-default-association-extensions/

Could this be related to issue I'm having: https://gist.github.com/2006057 ?