validación que tenga purchase_id para actualizar

purchase.py

@@ -9,11 +9,12 @@ class account_invoice(models.Model):
     def invoice_validate(self):
         res = super(account_invoice, self).invoice_validate()
         for item in self.invoice_line:
-            product = self.env['product.product'].browse(item.product_id.id)
-            if product:
-                template = self.env['product.template'].browse(product.product_tmpl_id.id)
-                if(template.price_change == 'purchase'):
-                    product.write({'standard_price': item.price_unit, 'last_purchase_date': date.today()})
-                else:
-                    product.write({'last_purchase_date': date.today()})
+            if item.purchase_line_id:
+                product = self.env['product.product'].browse(item.product_id.id)
+                if product:
+                    template = self.env['product.template'].browse(product.product_tmpl_id.id)
+                    if(template.price_change == 'purchase'):
+                        product.write({'standard_price': item.price_unit, 'last_purchase_date': date.today()})
+                    else:
+                        product.write({'last_purchase_date': date.today()})
         return res