diff --git a/backend/commerce/models.py b/backend/commerce/models.py
index 1e58abf..9f78dec 100644
--- a/backend/commerce/models.py
+++ b/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
 
diff --git a/backend/commerce/serializers.py b/backend/commerce/serializers.py
index 3a0e37f..21f599d 100644
--- a/backend/commerce/serializers.py
+++ b/backend/commerce/serializers.py
@@ -251,7 +251,7 @@ class OrderCreateSerializer(serializers.Serializer):
 
 			# přidame fieldy, které nejsou vyplněné
 			for field in required_fields: 
-				if attrs.get(field) not in required_fields:
+				if not attrs.get(field):
 					missing_fields.append(field)
 
 			if missing_fields:
@@ -307,10 +307,13 @@ class OrderCreateSerializer(serializers.Serializer):
 
 
 			# -- Slevové kódy --
+			# Discount codes need to be added before payment/final save because calculate_total_price uses them
 			if codes:
 				discounts = list(DiscountCode.objects.filter(code__in=codes))
 				if discounts:
 					order.discount.add(*discounts)
+					# Save to recalculate total with discounts
+					order.save(update_fields=["total_price", "updated_at"])
 
 
 
@@ -324,7 +327,7 @@ class OrderCreateSerializer(serializers.Serializer):
 
 			# přiřadíme k orderu
 			order.payment = payment
-			order.save(update_fields=["payment"])
+			order.save(update_fields=["payment", "updated_at"])
 			
 			return order
 
diff --git a/backend/vontor_cz/settings.py b/backend/vontor_cz/settings.py
index 54c266b..1c2a063 100644
--- a/backend/vontor_cz/settings.py
+++ b/backend/vontor_cz/settings.py
@@ -326,7 +326,10 @@ REST_FRAMEWORK = {
 	'DEFAULT_THROTTLE_RATES': {
 		'anon': '100/hour',   # unauthenticated
 		'user': '2000/hour',  # authenticated
-	}
+	},
+	
+    'EXCEPTION_HANDLER': 'trznice.utils.custom_exception_handler',
+    
 }
 #--------------------------------END REST FRAMEWORK 🛠️-------------------------------------
 
diff --git a/backend/vontor_cz/utils.py b/backend/vontor_cz/utils.py
new file mode 100644
index 0000000..dd8469e
--- /dev/null
+++ b/backend/vontor_cz/utils.py
@@ -0,0 +1,44 @@
+import os
+from datetime import datetime
+from rest_framework.fields import DateTimeField
+from django.conf import settings
+from django.core.exceptions import ValidationError as DjangoValidationError
+from rest_framework.views import exception_handler
+from rest_framework.exceptions import ValidationError as DRFValidationError
+
+
+def custom_exception_handler(exc, context):
+    """
+    Custom exception handler to convert Django ValidationError to DRF ValidationError (400 instead of 500)
+    """
+    # Convert Django ValidationError to DRF ValidationError
+    if isinstance(exc, DjangoValidationError):
+        if hasattr(exc, 'error_dict'):
+            # Multiple field errors
+            exc = DRFValidationError(detail=exc.message_dict)
+        elif hasattr(exc, 'error_list'):
+            # Single error or list of errors
+            exc = DRFValidationError(detail=exc.messages)
+        else:
+            # Fallback
+            exc = DRFValidationError(detail=str(exc))
+    
+    # Call REST framework's default exception handler
+    return exception_handler(exc, context)
+
+
+def truncate_to_minutes(dt: datetime) -> datetime:
+    return dt.replace(second=0, microsecond=0)
+
+
+class RoundedDateTimeField(DateTimeField):
+    def to_internal_value(self, value):
+        dt = super().to_internal_value(value)
+        return truncate_to_minutes(dt)
+    
+IMPORT_DIR = "data_imports"
+def get_imports_dir():
+    base_dir = settings.BASE_DIR  # adjust if your BASE_DIR is different
+    imports_dir = os.path.join(base_dir, IMPORT_DIR)
+    os.makedirs(imports_dir, exist_ok=True)
+    return imports_dir