Question

I have a django model:

class ActivationCode(models.Model):
    id = models.FloatField(primary_key=True)
    user = models.ForeignKey('User', to_field='id', on_delete=models.CASCADE)

I was having a hard time selecting an ActivationCode instance, so I tried to get the instance using its own id:

activation_codes = ActivationCode.objects.all()
print len(tuple(activation_codes))
>>> 1
for code in activation_codes:
    cid = code.id
ActivationCode.objects.get(id=cid)
>>> DoesNotExist: ActivationCode matching query does not exist.

What’s going on? Why does it say the object doesn’t exist when I’m using its own id? How come I can get the instance using .all() but not .get()?

It might be worth noting that the above code is running in a class-based view in a test environment (TestCase). When I try the same thing in the terminal, it works fine.

Answer 1

Floats as primary keys are a Bad Idea.

You cannot guarantee the float representation is exactly the same on all levels. Python float implementation is interpreter-specific and likely differs in precision from your DB doubles.