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 product_classes.id = 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 = #{user.id}"}}) do       find(:all, :select => "product_classes.*")     end 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   ... end

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.

Regards, John