[Python] Using Protocols in django models raises metaclass conflict error

Let's say I have PEP-544 protocol called Summable :

class Summable(Protocol):
@property
total_amount()-> Decimal:
...



And I have model Item that implements the Protocol

class Item(Summable, models.Model):
discount = models.DecimalField(
decimal_places=2,
validators=[MaxValueValidator(1)],
default=Decimal('0.00'),
max_digits=10
)
price = models.DecimalField(
decimal_places=4,
validators=[MinValueValidator(0)],
max_digits=10
)

@property
def total_amount(self) - > Decimal:
return self.price - self.price * self.discount

class Meta:
ordering = ['id']



I get:

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases


The same happens even if I extend Item's Meta from Summable.Meta and models.Model.Meta as well.

I am using python 3.9 Any ideas?

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