Versionise API and cleanups

uncloud_django_based/uncloud/uncloud_pay/models.py

@@ -825,7 +825,11 @@ class Order(models.Model):
 
     # Trigger initial bill generation at order creation.
     def save(self, *args, **kwargs):
-        super(Order, self).save(*args, **kwargs)
+        if self.ending_date and self.ending_date < self.starting_date:
+            raise ValidationError("End date cannot be before starting date")
+
+        super().save(*args, **kwargs)
+
         Bill.generate_for(self.starting_date.year, self.starting_date.month, self.owner)
 
     @property