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/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