eiru_purchase_lines_total/models/purchase.py

# -*- coding: utf-8 -*-
##############################################################################
from openerp import tools, api
from openerp import api, fields, models, _

class PickingTotalQty(models.Model):
    _inherit = 'purchase.order'

    tot_purchase_qty = fields.Float(compute='_calculate_purchase_qty', string='Total Cantidad Comprado', help="Total cantidad comprado")

    def _calculate_purchase_qty(self):
        for rs in self:
            sumqty = 0
            for line in rs.order_line:
                sumqty += line.product_qty
        rs.tot_purchase_qty = sumqty