Web Commerce - Serializers

def get_item_count(self, obj):
    """Total number of items in cart"""
    return sum(item.get('quantity', 0) for item in obj.line_items)

def get_subtotal(self, obj):
    """Calculate cart subtotal"""
    total = sum(float(item.get('total_price', 0)) for item in obj.line_items)
    return f"{total:.2f}"

def get_time_remaining(self, obj):
    """Calculate time remaining in seconds"""
    if obj.is_expired():
        return 0
    remaining = (obj.expires_at - timezone.now()).total_seconds()
    return max(0, int(remaining))

def validate(self, data):
    inventory = InventoryLevel.objects.get(
        variant=variant,
        store_id=data['store_id'],
        is_active=True
    )
    available = inventory.stock_on_hand - inventory.stock_reserved
    if available < data['quantity']:
        raise serializers.ValidationError(
            f"Insufficient inventory for '{sku}'. Available: {available}, Requested: {data['quantity']}"
        )

def validate_cart_id(self, value):
    cart = Cart.objects.get(cartId=value)
    if cart.status != Cart.Status.ACTIVE:
        raise ValidationError(f"Cart is {cart.status}, cannot checkout")
    if not cart.line_items:
        raise ValidationError("Cart is empty")
    return value

def validate(self, data):
    checkout_form_data = data.get('checkout_form_data') or {}
    payment_timing = checkout_form_data.get('payment_timing') or checkout.payment_timing

    if payment_timing == 'immediate' and not data.get('payment_method_id'):
        raise ValidationError({
            'payment_method_id': 'Payment method is required for immediate payment'
        })

cart = Cart.objects.get(cartId=cart_id)
serializer = CartSerializer(cart)
return Response(serializer.data)

# Response:
{
    "cartId": "abc-123",
    "status": "ACTIVE",
    "items": [
        {
            "variant_id": "var-456",
            "sku": "SKU-001",
            "name": "Size M / Blue",
            "quantity": 2,
            "unit_price": "49.99",
            "total_price": "99.98"
        }
    ],
    "item_count": 2,
    "subtotal": "99.98"
}

serializer = AddToCartSerializer(data=request.data)
serializer.is_valid(raise_exception=True)

# Validation ensures inventory is available
# before CartService.add_item() is called

serializer = CompleteCheckoutSerializer(data=request.data)
serializer.is_valid(raise_exception=True)

# Validation ensures:
# - Session exists and not expired
# - Payment method provided if immediate payment
# - Session not already completed