cascade_save does not save new referenced document

When saving a document with a ReferenceField holding a new object, I get an error:

mongoengine.errors.ValidationError: ValidationError 
You can only reference documents once they have been saved to the database

I naively expected cascade=True to automatically save the new document, while apparently it only automatically saves changes to existing documents.

Is this something that could be changed or is it meant to be for good reasons?

Example:

    class User(Document):
        name = StringField()

    class UserSubscription(Document):
        name = StringField()
        user = ReferenceField(User, dbref=False)

    User.drop_collection()
    UserSubscription.drop_collection()

    # u1 = User(name="Ross").save()
    u1 = User(name="Ross")

    sub = UserSubscription(user=u1).save()

I'd like this not to throw ValidationError but to save u1 in cascade.

Start with ReferenceField validation and the cascade=True save path described in the issue, using the User and UserSubscription example to trace where the unsaved reference is rejected. Determine the intended behavior for cascading new referenced documents and define tests showing whether the example saves successfully without breaking existing-reference behavior.