How to use the cachier.mongo_core._MongoCore._INDEX_NAME function in cachier

def test_mongo_index_creation():
    """Basic Mongo core functionality."""
    collection = _test_mongetter()
    _test_mongo_caching.clear_cache()
    val1 = _test_mongo_caching(1, 2)
    val2 = _test_mongo_caching(1, 2)
    assert val1 == val2
    assert _MongoCore._INDEX_NAME in collection.index_information()

def __init__(self, mongetter, stale_after, next_time):
        if 'pymongo' not in sys.modules:
            warnings.warn((
                "Cachier warning: pymongo was not found. "
                "MongoDB cores will not function."))
        _BaseCore.__init__(self, stale_after, next_time)
        self.mongetter = mongetter
        self.mongo_collection = self.mongetter()
        index_inf = self.mongo_collection.index_information()
        if _MongoCore._INDEX_NAME not in index_inf:
            func1key1 = IndexModel(
                keys=[('func', ASCENDING), ('key', ASCENDING)],
                name=_MongoCore._INDEX_NAME)
            self.mongo_collection.create_indexes([func1key1])

def __init__(self, mongetter, stale_after, next_time):
        if 'pymongo' not in sys.modules:
            warnings.warn((
                "Cachier warning: pymongo was not found. "
                "MongoDB cores will not function."))
        _BaseCore.__init__(self, stale_after, next_time)
        self.mongetter = mongetter
        self.mongo_collection = self.mongetter()
        index_inf = self.mongo_collection.index_information()
        if _MongoCore._INDEX_NAME not in index_inf:
            func1key1 = IndexModel(
                keys=[('func', ASCENDING), ('key', ASCENDING)],
                name=_MongoCore._INDEX_NAME)
            self.mongo_collection.create_indexes([func1key1])