@classmethod
def validate_token(cls, user_id, subject, token):
    """Checks for existence of a token, given user_id, subject and token.

        :param user_id:
            User unique ID.
        :param subject:
            The subject of the key. Examples:

            - 'auth'
            - 'signup'
        :param token:
            The token string to be validated.
        :returns:
            A :class:`UserToken` or None if the token does not exist.
        """
    return cls.token_model.get(user=user_id, subject=subject, token=token)
