'Look for a given location in the collection. If the item is not present, raise
ItemNotFoundError.'

@autoretry_read()
def _find_one(self, location):

    assert isinstance(location, UsageKey)
    item = self.collection.find_one({'_id': location.to_deprecated_son()})
    if (item is None):
        raise ItemNotFoundError(location)
    return item
