@classmethod
def filter_field(cls):
    """Return a field that can be used as a unique key for this Resource.

        :returns: A field that can be used as a unique key.
        :rtype: str
        """
    if cls._filter_field is None:
        with cls._filter_field_lock:
            if cls._filter_field is None:
                cls._filter_field = cls._schema().filterable_key()
    return cls._filter_field
