get-or-create

[jesteria]

The item manager should provide a method encapsulating the non-trivial logic of get-or-create, a la:

try:
    return get(…)
except DoesNotExist:
    item = Item(…)
    try:
        item.save()
    except ConditionalCheckFailedException:
        exc_info = sys.exc_info()
        try:
            return get()
        except DoesNotExist:
            six.reraise(*exc_info)
    else:
        return item