Add a field in the index of the model.

    Args:
        fieldname (Text): This parameters register a new field in specified model.
        fieldspec (Name, optional): This option adds various options as were described before.

    Returns:
        TYPE: The new schema after deleted is returned.

def add_field(self, fieldname, fieldspec=whoosh_module_fields.TEXT):
    """Add a field in the index of the model.

    Args:
        fieldname (Text): This parameters register a new field in specified model.
        fieldspec (Name, optional): This option adds various options as were described before.

    Returns:
        TYPE: The new schema after deleted is returned.
    """
    self._whoosh.add_field(fieldname, fieldspec)
    return self._whoosh.schema