Issues adding a field to POS Order report

Hi, I am trying to put *'Product Category'* to the *POS Order Analysis*. Still very new to OpenERP. So far I did: In ***'pos_order_report.py'*** add 'categ_id': fields.many2one('product.category','Category of Product', readonly=True), to the *pos_order_report* class, and then pt.categ_id as categ_id, ... group by ... pt.categ_id in the SQL In ***'pos_order_report_view.xml'*** add in the *view_report_pos_order_tree* and in *view_report_pos_order_search* The Error it shows when I try to filter by "Category of Product" is: > ProgrammingError: Column report_pos_order.categ_id does not exist> LINE 1: ...HERE (("report_pos_order"."year" = '2013') AND ("report_po... What am I doing wrong?