检查对象是否存在

小开

最佳答案

Since filter returns a QuerySet, you can use count to check how many results were returned. This is assuming you don't actually need the results.

num_results = User.objects.filter(email = cleaned_info['username']).count()

After looking at the documentation though, it's better to just call len on your filter if you are planning on using the results later, as you'll only be making one sql query:

A count() call performs a SELECT COUNT(*) behind the scenes, so you should always use count() rather than loading all of the record into Python objects and calling len() on the result (unless you need to load the objects into memory anyway, in which case len() will be faster).

num_results = len(user_object)

小开

You can also use get_object_or_404(), it will raise a Http404 if the object wasn't found:

user_pass = log_in(request.POST) #form class
if user_pass.is_valid():
cleaned_info = user_pass.cleaned_data
user_object = get_object_or_404(User, email=cleaned_info['username'])
# User object found, you are good to go!
...

小开

the boolean value of an empty QuerySet is also False, so you could also just do...

...
if not user_object:
do insert or whatever etc.

小开

I think the easiest from a logical and efficiency point of view is using the queryset's exists() function, documented here:

https://docs.djangoproject.com/en/stable/ref/models/querysets/#django.db.models.query.QuerySet.exists

So in your example above I would simply write:

if User.objects.filter(email = cleaned_info['username']).exists():
# at least one object satisfying query exists
else:
# no object satisfying query exists

小开

You can use:

try:
# get your models
except ObjectDoesNotExist:
# do something

小开

If the user exists you can get the user in user_object else user_object will be None.

try:
user_object = User.objects.get(email = cleaned_info['username'])
except User.DoesNotExist:
user_object = None
if user_object:
# user exist
pass
else:
# user does not exist
pass