from openerp.osv import fields, osv
import openerp.addons.decimal_precision as dp

class product_dim_price(osv.osv):
    _inherit = "product.product"

    def _get_dim_price(self, cr, uid, ids, field_name=None, arg=False, context=None):
        res = {}
        for prod in self.browse(cr, uid, ids, context=context):
            res[prod.id] = prod.length * prod.width
        return res

    _columns = {
        'dim_price': function(_get_dim_price, digits_compute=dp.get_precision('Product Unit of Measure'),
            type='float', string='Dimension Price'),
    }
product_dim_price()