I'm using OpenERP V7.  I needed to make the field product_product.qty_available sortable, so I inherited the product.product class and added the store=True to the field definition. Along the way I copied the function _product_available to my inherited class so the code does not crash. I know that this does not cope with the OO concepts

Now I need help in the following two points

1. how to make a function field sortable without appealing to store the field in the database

2. when inheriting a function field, how to do so without copying the function that calculates its value