beanie: BUG: fetch_links=True does not work

Hi,

I am trying to retrieve documents from the DB (using the “get” classmethod) and I want to retrieve the linked documents as well. However, that does not work and I get the following error:

FieldPath field names may not start with '$'., full error: 
{'operationTime': Timestamp(1651142380, 1), 'ok': 0.0, 
'errmsg': "FieldPath field names may not start with '$'.", 'code': 16410, 'codeName': 
'Location16410', '$clusterTime': {'clusterTime': Timestamp(1651142380, 1), 
'signature': {'hash': b'|Cnf\xb8\xdev,\xf4\x9c\xf6\x92\x0f\xfcX\xbcy\x8a1\xf0', 'keyId': 7061329875650478083}}}

If I try with fetch_links=False, I can successfully get the object back, but without getting the data for the linked document (instead I get a Link object of the linked class). Same is true for find and find_many - they are not able to fetch the links without raising the error.

Does not neccessarily seem to be a Beanie problem, but have you experienced this before?