Django check if a related object exists error: RelatedObjectDoesNotExist

Use hasattr(self, 'customers') to avoid the exception check as recommended in Django docs:

def has_related_object(self):
    return hasattr(self, 'customers') and self.car is not None

Although it's an old question, I thought this can be helpful for someone looking to handle this type of exception, especially when you want to check for OneToOne relations.

My solution is to use ObjectDoesNotExist from django.core.exceptions:

from django.core.exceptions import ObjectDoesNotExist

class Business(base):
      name =  models.CharField(max_length=100, blank=True, null=True)

    def has_related_object(self):
        try:
            self.customers
            self.car
            return True
        except ObjectDoesNotExist:
            return False


class Customer(base):
      name =  models.CharField(max_length=100, blank=True, null=True)
      person = models.OneToOneField('Business', related_name="customer")

This is because the ORM has to go to the database to check to see if customer exists. Since it doesn't exist, it raises an exception.

You'll have to change your method to the following:

def has_related_object(self):
    has_customer = False
    try:
        has_customer = (self.customers is not None)
    except Customer.DoesNotExist:
        pass
    return has_customer and (self.car is not None)

I don't know the situation with self.car so I'll leave it to you to adjust it if it needs it.

Side note: If you were doing this on a Model that has the ForeignKeyField or OneToOneField on it, you would be able to do the following as a shortcut to avoid the database query.

def has_business(self):
    return self.business_id is not None