@staticmethod
def unset_update(keys):
    """
        Creates a delete query to remove certain keys from a document or list of documents.
        :param keys: a list of keys to delete (or a single key)
        :return: a dictionary containing a proper/valid update statement for MongoDB.
        """
    unset_update = {'$unset': {}}
    if isinstance(keys, str):
        unset_update[keys] = 1
    elif isinstance(keys, list):
        for key in keys:
            unset_update[key] = 1
    return unset_update
