Django: check whether an object already exists before adding

How do I check whether an object already exists, and only add it if it does not already exist?

Here's the code - I don't want to add the follow_role twice in the database if it already exists. How do I check first? Use get() maybe - but then will Django complain if get() doesn't return anything?

cur开发者_如何学运维rent_user = request.user
follow_role = UserToUserRole(from_user=current_user, to_user=user, role='follow')
follow_role.save()

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.

If you're using a recent version of Django, you can use the unique_together option to the UserToUserRole model, and then use the get_or_create() method Joe Holloway showed. This will guarantee that you can't get a duplicate.

If you are looking for a bool value

UserToUserRole.objects.filter(
    from_user=current_user, to_user=user, role='follow'
).exists()

You can use get(), but you'll have to wrap it with try-except like this:

try:
    obj = UserToUserRole.objects.get(from_user=current_user, to_user=user, role='follow')
except UserToUserRole.DoesNotExist:
    # here write a code to create a new object

If you want the check done every time before save you could use the pre save signal to check, http://docs.djangoproject.com/en/dev/ref/signals/#django.db.models.signals.pre_save

Alternatively in the model you could specify unique=True for the property that determines whether an item is the same item, this will cause an Exception (django.db.IntegrityError) to be thrown if you try to save the same thing again.

If you have more than one field together that makes something unique you'll need to use unique_together in the Meta inner class http://docs.djangoproject.com/en/dev/ref/models/options/#unique-together