Check if a OneToOne relation exists in Django

So you have a least two ways of checking that.First is to create try/catch block to get attribute, second is to use hasattr.

class A(models.Model):   def get_B(self):       try:          return self.b       except:          return Noneclass B(models.Model):   ref_a = models.OneToOneField(related_name='ref_b', null=True)

Please try to avoid bare except: clauses. It can hide some problems.

The second way is:

class A(models.Model):    def get_B(self):       if(hasattr(self, 'b')):           return self.b       return Noneclass B(models.Model):    ref_a = models.OneToOneField(related_name='ref_b', null=True)

In both cases you can use it without any exceptions:

a1 = A.objects.create()a2 = A.objects.create()b1 = B.objects.create()b2 = B.objects.create(ref_a=a2)# then I call:print(a1.get_b)  # No exception raisedprint(a2.get_b)  # returns b2print(b1.a)  # returns Noneprint(b2.a)  # returns a2

There is no other way, as throwing the exception is default behaviour from Django One to One relationships.

And this is the example of handling it from official documentation.

>>> from django.core.exceptions import ObjectDoesNotExist>>> try:>>>     p2.restaurant>>> except ObjectDoesNotExist:>>>     print("There is no restaurant here.")There is no restaurant here.

Individual model classes provide a more specific exception called DoesNotExist that extends ObjectDoesNotExist. My preference is to write it this way:

b = Nonetry:    b = a.ref_bexcept B.DoesNotExist:    pass

hasattr works fine with Django1.11 !You may use getattr for shorter version:

getattr(self, 'field', default)

In your case

b = getattr(a, 'ref_b', None)

https://docs.python.org/3.6/library/functions.html#getattr