Go over all attributes in `model` and add :class:`SourceString` to them.

    Args:
        model (obj): :class:`Model` instance.

    Returns:
        obj: :class:`Model` instance with :class:`SourceString` descriptors.

def _add_source(model):
    """
    Go over all attributes in `model` and add :class:`SourceString` to them.

    Args:
        model (obj): :class:`Model` instance.

    Returns:
        obj: :class:`Model` instance with :class:`SourceString` descriptors.
    """
    ignored_keys = {"author_tags", "original_xml", "additional_info"}

    # convert all values to source strings
    source = "Aleph"
    for key, val in model.get_mapping().iteritems():
        if key in ignored_keys:
            continue

        if type(val) in [list, tuple]:
            ss_val = [
                SourceString(item, source).to_dict()
                for item in val
            ]
        else:
            ss_val = [SourceString(val, source).to_dict()]

        setattr(model, key, ss_val)

    return model