#property doesn't store value in DB - django

Question

My model is like this:
class Cart(models.Model):
id = models.AutoField(primary_key=True)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField(null=False,blank=False)
total = models.FloatField(null=False,blank=False)
#property
def total(self):
return self.quantity * self.product.price
The total appears in my admin dashboard but when I inspect the db from the shell I don't get the total value, I get the other values:
<QuerySet [{'id': 42, 'product_id': 11, 'quantity': 2}]>
Expected output:
<QuerySet [{'id': 42, 'product_id': 11, 'quantity': 2, 'total': 29.00}]>

Answer 1

The #property decorator is a Python method, your database knows nothing about it. The property will still work in Python code and Django templates, but you won't be able to use DB operations against it like other fields (e.g. using .filter or .values with total).
If you need to query against the total, what you probably want is to use annotations instead.
Just omit the total from your model altogether and you can annotate your queries to have the DB calculate the total on-the-fly:
from django.db.models import ExpressionWrapper, F
queryset = Cart.objects.annotate(
total=ExpressionWrapper(
F("quantity") * F("product__price"), output_field=models.FloatField()
)
)
for cart in queryset:
print(cart.total)
ExpressionWrapper is used to define the output_field (so Django knows what type the result should be) which is needed when multiplying integerfields by floatfields.
If you want queries to always return the annotated total, you can override the model's default manager.
class CartManager(models.Manager):
def with_total(self):
qs = super().get_queryset()
return qs.annotate(
total=ExpressionWrapper(
F("quantity") * F("product__price"), output_field=models.FloatField()
)
)
def get_queryset(self): # override the default queryset
return self.with_total()
class Cart(models.Model):
objects = CartManager() # override the default manager
id = models.AutoField(primary_key=True)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField(null=False, blank=False)

Answer 2

Django models save data once the save() method execuate.
Try this, it works very well. It's a best practice
class Cart(models.Model):
id = models.AutoField(primary_key=True)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField(null=False,blank=False)
total = models.FloatField(null=False,blank=False)
def save(self, *args, **kwargs):
self.total = self.quantity * self.product.price
super(Cart, self).save(*args, **kwargs)

Answer 3

#property decorator is not for saving data in DB.
What the #property decorator does is declare that it can be accessed as a regular property.
This means you can call total as if it were a member your model instead of a function, so like this:
total = cart.total #in views
{{ cart.total }} #in templates
instead of
total = cart.total() #in views
{{ cart.total() }} #in templates
for more details on property, you can refer here
If you still want to store the value in total field, then save it to database instead of returning it in the method.
class Cart(models.Model):
id = models.AutoField(primary_key=True)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField(null=False,blank=False)
total = models.FloatField(null=False,blank=False)
#property
def save_total(self):
total = self.quantity * self.product.price
self.total = total
return total
or
you can save the data to total in your views, while your cart is saved in view.py