Check if `key` contains ".", "$" or null characters.

       https://docs.mongodb.com/manual/reference/limits/#Restrictions-on-Field-Names

        Args:
            obj_name (str): object name to use when raising exception
            key (str): key to validated

        Returns:
            None: validation successful

        Raises:
            ValidationError: will raise exception in case of regex match.

def validate_key(obj_name, key):
    """Check if `key` contains ".", "$" or null characters.

       https://docs.mongodb.com/manual/reference/limits/#Restrictions-on-Field-Names

        Args:
            obj_name (str): object name to use when raising exception
            key (str): key to validated

        Returns:
            None: validation successful

        Raises:
            ValidationError: will raise exception in case of regex match.
    """
    if re.search(r'^[$]|\.|\x00', key):
        error_str = ('Invalid key name "{}" in {} object. The '
                     'key name cannot contain characters '
                     '".", "$" or null characters').format(key, obj_name)
        raise ValidationError(error_str)