Question

I have two models like this:

class Collar(models.Model):
    num_tags = models.BigIntegerField()

class Dog(models.Model):
    num_legs = models.BigIntegerField()
    collar =  models.OneToOneField(Collar,null=True,blank=True)

Whenever I try to do something like:

dog = Dog.objects.all()[0]
if dog.collar:
    #do something ...

I get a DoesNotExist Error on the line: if dog.collar.

How can I tell Django that this is ok? That a collar won’t always exist and I want to check for its existence without triggering an error?

Answer 1

Use a try..except block:

try:
   if dog.collar:
       # Do something
except Collar.DoesNotExist:
   # Do something else

Also, never assume that all() will always return something. In ideal circumstances, sure, but life is never ideal. You need to catch the potential IndexError with a statement like that, and have a contingency plan in place:

try:
    dog = Dog.ojbects.all()[0]
except IndexError:
    # Contingency plan so website doesn't explode