Check if a OneToOne relation exists in Django
Solution 1
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 None
class 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 None
class 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 raised
print(a2.get_b) # returns b2
print(b1.a) # returns None
print(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.
Solution 2
Individual model classes provide a more specific exception called DoesNotExist that extends ObjectDoesNotExist. My preference is to write it this way:
b = None
try:
b = a.ref_b
except B.DoesNotExist:
pass
Solution 3
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
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Comments
-
Alfred Huang almost 4 years
Now I'm using django 1.6
I have two models relates with a
OneToOneField
.class A(models.Model): pass class B(models.Model): ref_a = models.OneToOneField(related_name='ref_b', null=True)
First see my code that points out the problem:
a1 = A.objects.create() a2 = A.objects.create() b1 = B.objects.create() b2 = B.objects.create(ref_a=a2) # then I call: print(a1.ref_b) # DoesNotExist Exception raised print(a2.ref_b) # returns b2 print(b1.ref_a) # returns None print(b2.ref_a) # returns a2
Now the problem is, if I want to check a
A
object, to judge whether it exists aB
objects referencing it. How can I do?The valid way I tried is only to try and catch an exception, but is there any other prettier way?
My effort:
1 - The below code works, but is too ugly!
b = None try: b = a.ref_b except: pass
2 - I also tried to check the attributes in a, but not working:
b = a.ref_b if hasattr(a, 'ref_b') else None
Do you meet the same problem, friends? Please point me a way, thank you!
-
Yogesh dwivedi Geitpl over 9 yearsI think this will work for you, please keep in mind change your class name at least in 2 later. b = hasattr(a_class_object, 'b class') and a.ref_b or None. I am using same and working for me.
-
Alfred Huang over 9 years@Yogesh I don't quite catch, could you please post a more detail solution?
-
beruic almost 7 years
hasattr
seems to work fine for me on Django 1.8.17 with Python 2.
-
-
Duda Nogueira over 8 yearsGreat answer! Thanks!
-
user2230901 over 6 yearswhy not put
b = None
in theexcept
clause? (to save a line)