factory_reference_invoice/account_invoice.py

from openerp import models,api,fields

class AccountInvoiceLine(models.Model):
    _inherit = 'account.invoice.line'
    _name = 'account.invoice.line'

    @api.one
    @api.depends('product_id')
    def _get_factory_ref(self):
        self.product_factory_ref = self.product_id.factory_reference

    product_factory_ref = fields.Char(string="Referencia de Fabrica", compute="_get_factory_ref")