In our latest project we had to implement condition-based categories. (like a category which holds all products with a price greater than X).

We serialized the condition-hash into a 'category_conditions' column and implemented a proxy-association method to evaluate the condition and return a scope (to keep method-chaining working):

   1  class Category < ActiveRecord::Base
   2    has_many :products do
   3      def with_condition
   4        scoped( { :conditions => proxy_owner.category_conditions } )
   5      end
   6    end
   7  end

No we can use it like this:

   1  c = Category.create( :category_condition => ["price >= ?", 99] )
   2  c.products # => regular associated products
   3  c.products.with_condition # => products with a price greater than 99