item_frequencies() always returns {None: $count} on EmbeddedDocumentListField

Hiya!

While using item_frequencies() on an EmbeddedDocumentListField field, the return value seems to be {None: $count } in all cases. The document below is an example in the Sigma collection. Maybe I'm forgetting something or is this an unsupported feature? 🤔

Python version: Python 3.8.6
MongoEngine version: mongoengine==0.20.0

Example Document

hash:"f8127c4e4cdaa4b551195a2b13394a88b0d1e8748c587ad1e122df03e2b1120e"
date:2019-12-19T00:00:00.000+00:00
level: "high"
description:"Detects download of Ursnif malware done by dropper documents."
techniques: [
  {
     name: "Web Service",
     ...
  }
]

Command used

Sigma.objects().item_frequencies('techniques.name')

Return value

{None: 734}

Note

Running item_frequencies() on a root field such as 'level' in the document does work and returns:

# Output of Sigma.objects().item_frequencies('level')
{'critical': 143, 'high': 349, 'low': 58, 'medium': 184}

Start by reproducing the issue with an EmbeddedDocumentListField and item_frequencies('techniques.name'), then inspect the item_frequencies implementation and its existing tests. Done means nested fields in embedded-document lists return frequencies for their actual values instead of {None: count}, while root-field behavior remains unchanged.