Django: Check Whether an Object Already Exists Before Adding

Django: check whether an object already exists before adding

There's a helper function for this idiom called 'get_or_create' on your model manager:

role, created = UserToUserRole.objects.get_or_create(
    from_user=current_user, to_user=user, role='follow')

It returns a tuple of (model, bool) where 'model' is the object you're interested in and 'bool' tells you whether it had to be created or not.

Django - Checking if objects exists and raising error if it does

The problem is get returns an object, but exists only works with querysets. A Model.DoesNotExist error will be raised when you use get and the object does not exist.

You should use filter instead of get:

qs = SomeModel.objects.filter(quote_number = quote)
if qs.exists():
    ...
else:
    ...

Checking for object if it already exists in manytomany field

you are comparing User object to User's name (string).

value == User.objects.get(pk=user_id).name

which will fail always.Try this:

value.name == User.objects.get(pk=user_id).name

Check if an object exists

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)

Check if Many to Many exists in django?

We can prevent fetching the related student of the user in case we want to remove the university from the wishlist. This is usually more safer since a User that has no related student will then not raise an error:

university = University.objects.get(id=pk)
if university.student_set.filter(user=request.user).exists():
    Student.wishlist.through.objects.filter(
        university=university,
        student__user=request.user
    ).delete()
else:
    request.user.student.wishlist.add(university)

We can slightly boost efficiency in the .remove(...) path by not fetching the .student of the user, and thus implement this as:

Identifying if object already exists in CreateView

So, the solution I implemented had to go to the ModelForm:

class LabelForm(forms.ModelForm):

    class Meta:
        model = Label
        fields = ('name',)

    def clean(self):
        if Label.objects.filter(name=self.cleaned_data['name'].lower()).exists()
            raise forms.ValidationError('Label exists!')

        return self.cleaned_data

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