named_scope and habtm chaining problem

Can anyone help me with this?

I have a habtm relationship. I defined a named_scope on one side of
it. It works fine except when I chain it. For example:

class Category < ActiveRecord::Base
  has_and_belongs_to_many :items

class Item < ActiveRecord::Base
  has_and_belongs_to_many :categories

  named_scope :cat, lambda {|cat| {:include
=> :categories, :conditions => [" = ?", cat]}}

Simple. Easy. Normal, right?

But, let's see I have the following in my join table
category_id | item_id