purchase_order_line_total/models/sale_purchase_discount.py

# -*- encoding: utf-8 -*-
from openerp import models, api, fields
from pytz import timezone
from openerp.tools import DEFAULT_SERVER_DATETIME_FORMAT, DEFAULT_SERVER_DATE_FORMAT
from datetime import datetime,timedelta

DATE_FORMAT = '%Y-%m-%d'

class SalePurchaseDiscount(models.Model):
    _name = 'sale.purchase.discount'

    name = fields.Char('Nombre')
    value = fields.Float(string='Valor',default=0.0)
    type = fields.Selection([('sale','Ventas'),('purchase','Compras')],'Tipo de descuento')

class PurchaseOrderLine(models.Model):
    _inherit = 'purchase.order.line'
    _name = 'purchase.order.line'

    discount_id = fields.Many2one(
        'sale.purchase.discount',
        string='Descuento',
        ondelete='restrict'
    )

    @api.onchange('discount_id')
    def _calculate_purchase_discount(self):
        for line in self:
            line.discount = line.discount_id.value