EmbeddedDocumentListField cannot have a primary/unique field?

class RatingData(EmbeddedDocument):
    id = StringField(unique=True)
    rating = IntField()


class Subject(Document):
    id = IntField(primary_key=True)
    rating = EmbeddedDocumentListField(RatingData)

if not Subject.objects(id=par_subject_id).first() or not Subject.objects(
        id=par_subject_id).first().rating:
    Subject.objects(id=par_subject_id).upsert_one(rating=[rating_data])
else:
    Subject.objects(id=par_subject_id).update_one(push__rating=rating_data)

But there is duplicates in Mongo.

There is the index

Mongoengine 0.18.2
Python 3.8.0

Reproduce the Subject and RatingData example on MongoEngine 0.18.2 with MongoDB, starting from EmbeddedDocumentListField and the unique=True and primary_key declarations. Compare the behavior of upsert_one and push__rating with the displayed index. Done means establishing whether uniqueness is expected for embedded list fields and recording the confirmed behavior or required change with a regression test.