[FIX] sale_purchase: Fix wrong constraint definition

constraint validation should work correctly on multi records
creation/updates

addons/sale_purchase/models/product_template.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
-from odoo import api, fields, models
+from odoo import api, fields, models, _
 from odoo.exceptions import ValidationError
 
 
@@ -18,9 +18,11 @@ class ProductTemplate(models.Model):
     ]
 
     @api.constrains('service_to_purchase', 'seller_ids')
-    def validate_service_to_purchase(self):
-        if self.service_to_purchase and not self.seller_ids:
-            raise ValidationError("Please define the vendor from whom you would like to purchase this service automatically.")
+    def _check_service_to_purchase(self):
+        for template in self:
+            if template.service_to_purchase and not template.seller_ids:
+                raise ValidationError(_(
+                    "Please define the vendor from whom you would like to purchase this service automatically."))
 
     @api.depends('type', 'expense_policy')
     def _compute_service_to_purchase(self):