Using with_scope inside an association extension

Hi All,
I've created an association extension for a habtm association proxy. Inside of this extension I want to add some more joins.

module CategoryProductFinder
  def sold_by(user)
    joins = "INNER JOIN categories_product_classes ON = categories_product_classes.product_class_id"
    joins << " INNER JOIN sellers_product_classes ON categories_product_classes.product_class_id = sellers_product_classes.product_class_id"

    self.with_scope({:find => {:joins => joins, :conditions => "sellers_product_classes.user_id = #{}"}}) do
      find(:all, :select => "product_classes.*")
    end end

My category class has the habtm association proxy with extension included

class Category < ActiveRecord::Base
  has_and_belongs_to_many :product_classes, :extend => CategoryProductFinder

The intention is that this would allow me to write the following code

#assume user, and category are valid
my_products = category.product_classes.sold_by(user)

*My Problem*

Unfortunately it doesn't work. The :joins inside the with_scope replaces the joins for the habtm association proxy. Does anyone know how to obtain the original joins SQL inside a scoped method. I could hardcode it but I imagine there's a more elegant way.