How to handle possible race-condition in overwritten Document.save method - Mongoengine

Question

I am using flask-mongoengine and think I am running in some kind of race conditions while trying to overwrite the Document.save method.

My models (simplified) look like this:

class User(Document):
    meta = {"collection": "users"}
    name = StringField()


class Group(Document):
    meta = {"collection": "groups"}
    name = StringField()


class History(EmbeddedDocument):
    key = StringField()
    oldValue = StringField()
    newValue = StringField()


class Asset(DynamicDocument):
    meta = {"collection": "assets"}
    c_id = SequenceField()
    name = StringField()
    history = ListField(EmbeddedDocumentField(History))
    user = ReferenceField('User')
    group = ReferenceField('Group', required=True, default=Group.objects.first())

    def save(self, **kwargs):
        for key, value in self._data.items():
            history_update = History(
                key=key,
                oldValue="",
                newValue=str(value)
            )
            self.history.append(history_update)
        return super(Asset, self).save(**kwargs)

What I am trying to achieve is:

When a new Document of type Asset is created, add an entry of type History for each Key/Value pair of the document that changed. (Here from None to some value, I have similar code in the update method for changes on existing assets). This history list should be something like a changelog of the particular asset through its lifetime.

My problem with the current implementation is that:

1. c_id of type SequenceField is None in my for-loop.
2. str(value) for the User object gives me the correct user-object (or the result of my custom __str__ method) but str(value) for the Group object gives me DBRef('groups', '<mongoidstring>') and does not trigger my customer str method
3. When debugging with a breakpoint beforehand, these two errors do not occur. c_id has its correct value and my group object is a group object and not a DBRef object

I've tried saving the Document once before and then adding my history which at least gives me a correct c_id but the group is still a DBRef.

I do think the SequenceField is populated in parallel and therefore still None when I try to access it but not when I come through the debugger. But the DBRef still gives me headaches. And that I don't really see a way to properly implement my ChangeHistory through overwriting the save method. Any ideas how to properly handle this?