QuerySet for related, undefined yet Document?

Hello. I try to figure out, how to achieve this kind of task:

class Cart(Document):
    # some fields
    @queryset_manager
    def items(self, queryset):
        return QuerySet('CartItem').objects(cart=self)

class CartItem(Document):
    # some fields
    cart = ReferenceField(Cart)

without changing an order of class definition? Like it works with ReferenceField('DocumentStringName')

Start by tracing the queryset_manager and ReferenceField behavior shown in the issue, especially how each resolves document classes and string references. Determine whether the requested class-order independence is supported or requires a design decision; done means the documented example works without changing the class definition order, with coverage for the forward reference case.