How do I get the object if it exists, or None if it does not exist?
When I ask the model manager to get an object, it raises DoesNotExist when there is no matching object. go = Content.objects.get(name="baby") Instead of DoesNotExist, how can I have go be None instead?
There is no ‘built in’ way to do this. Django will raise the DoesNotExist exception every time.
The idiomatic way to handle this in python is to wrap it in a try catch:
try: go = SomeModel.objects.get(foo='bar') except SomeModel.DoesNotExist: go = None
What I did do, is to subclass models.Manager, create a
safe_get like the code above and use that manager for my models. That way you can write:
Since django 1.6 you can use first() method like so:
DoesNotExistexception if an object is not found for the given parameters. This exception is also an attribute of the model class. The
DoesNotExistexception inherits from
You can catch the exception and assign
None to go.
from django.core.exceptions import ObjectDoesNotExist try: go = Content.objects.get(name="baby") except ObjectDoesNotExist: go = None
You can create a generic function for this.
def get_or_none(classmodel, **kwargs): try: return classmodel.objects.get(**kwargs) except classmodel.DoesNotExist: return None
Use this like below:
go = get_or_none(Content,name="baby")
go will be None if no entry matches else will return the Content entry.
Note:It will raises exception MultipleObjectsReturned if more than one entry returned for name=”baby”
You can do it this way:
go = Content.objects.filter(name="baby").first()
Now go variable could be either the object you want or None
To make things easier, here is a snippet of the code I wrote, based on inputs from the wonderful replies here:
class MyManager(models.Manager): def get_or_none(self, **kwargs): try: return self.get(**kwargs) except ObjectDoesNotExist: return None
And then in your model:
class MyModel(models.Model): objects = MyManager()
Now you have MyModel.objects.get() as well as MyModel.objetcs.get_or_none()
you could use
exists with a filter:
Content.objects.filter(name="baby").exists() #returns False or True depending on if there is anything in the QS
just an alternative for if you only want to know if it exists
Handling exceptions at different points in your views could really be cumbersome..What about defining a custom Model Manager, in the models.py file, like
class ContentManager(model.Manager): def get_nicely(self, **kwargs): try: return self.get(kwargs) except(KeyError, Content.DoesNotExist): return None
and then including it in the content Model class
class Content(model.Model): ... objects = ContentManager()
In this way it can be easily dealt in the views i.e.
post = Content.objects.get_nicely(pk = 1) if post: # Do something else: # This post doesn't exist
It’s one of those annoying functions that you might not want to re-implement:
from annoying.functions import get_object_or_None #... user = get_object_or_None(Content, name="baby")
If you want a simple one-line solution that doesn’t involve exception handling, conditional statements or a requirement of Django 1.6+, do this instead:
x = next(iter(SomeModel.objects.filter(foo='bar')), None)
I think it isn’t bad idea to use
from django.shortcuts import get_object_or_404 def my_view(request): my_object = get_object_or_404(MyModel, pk=1)
This example is equivalent to:
from django.http import Http404 def my_view(request): try: my_object = MyModel.objects.get(pk=1) except MyModel.DoesNotExist: raise Http404("No MyModel matches the given query.")
You can read more about get_object_or_404() in django online documentation.
From django 1.7 onwards you can do like:
class MyQuerySet(models.QuerySet): def get_or_none(self, **kwargs): try: return self.get(**kwargs) except self.model.DoesNotExist: return None class MyBaseModel(models.Model): objects = MyQuerySet.as_manager() class MyModel(MyBaseModel): ... class AnotherMyModel(MyBaseModel): ...
The advantage of “MyQuerySet.as_manager()” is that both of the following will work:
Here’s a variation on the helper function that allows you to optionally pass in a
QuerySet instance, in case you want to get the unique object (if present) from a queryset other than the model’s
all objects queryset (e.g. from a subset of child items belonging to a parent instance):
def get_unique_or_none(model, queryset=None, **kwargs): """ Performs the query on the specified `queryset` (defaulting to the `all` queryset of the `model`'s default manager) and returns the unique object matching the given keyword arguments. Returns `None` if no match is found. Throws a `model.MultipleObjectsReturned` exception if more than one match is found. """ if queryset is None: queryset = model.objects.all() try: return queryset.get(**kwargs) except model.DoesNotExist: return None
This can be used in two ways, e.g.:
obj = get_unique_or_none(Model, **kwargs)as previosuly discussed
obj = get_unique_or_none(Model, parent.children, **kwargs)
if SomeModel.objects.filter(foo='bar').exists(): x = SomeModel.objects.get(foo='bar') else: x = None
Using an exception:
try: x = SomeModel.objects.get(foo='bar') except SomeModel.DoesNotExist: x = None
There is a bit of an argument about when one should use an exception in python. On the one hand, “it is easier to ask for forgiveness than for permission”. While I agree with this, I believe that an exception should remain, well, the exception, and the “ideal case” should run without hitting one.
- Database Administration Tutorials
- Programming Tutorials & IT News
- Linux & DevOps World
- Entertainment & General News
- Games & eSport