def _polymorph(cls, dct):
    """Just create a proper instance for this dct."""
    if '_id' not in dct or '/' not in dct['_id']:
        raise TypeError('Cannot infer document type!', dct)
    collection, _ = dct['_id'].split('/')
    if collection not in cls.__documents__:
        raise TypeError('Unknown document type!', collection)
    return cls.__documents__[collection](dct)
