Add custom exception handler and improve order logic

Introduced a custom DRF exception handler to convert Django ValidationError to DRF ValidationError and registered it in settings. Improved Order model's calculate_total_price method to avoid accessing related fields before the object is saved. Updated OrderCreateSerializer to save the order after adding discounts and payment, ensuring total price is recalculated. Added utility functions and a rounded DateTime field in a new utils.py.

backend/commerce/models.py

@@ -212,7 +212,8 @@ class Order(models.Model):
     def calculate_total_price(self):
         carrier_price = self.carrier.get_price() if self.carrier else Decimal("0.0")
 
-        if self.discount.exists():
+        # Check if order has been saved (has an ID) before accessing many-to-many relationships
+        if self.pk and self.discount.exists():
             discounts = list(self.discount.all())
 
             total = Decimal('0.0')
@@ -227,8 +228,10 @@ class Order(models.Model):
             total = Decimal('0.0')
             # getting all prices from order items (without discount) - using VAT-inclusive prices
             
-            for item in self.items.all():
-                total = total + (item.product.get_price_with_vat() * item.quantity)
+            # Only try to access items if order has been saved
+            if self.pk:
+                for item in self.items.all():
+                    total = total + (item.product.get_price_with_vat() * item.quantity)
 
             return total + carrier_price