Subclass of mongoengine's GenericEmbeddedDocumentField. Allows GenericEmbeddedDocumentField to read a custom field instead of _cls to construct the EmbeddedDocument.

fields.py

from mongoengine.fields import GenericEmbeddedDocumentField as MDBGenericEmbeddedDocumentField

class GenericEmbeddedDocumentField(MDBGenericEmbeddedDocumentField):
    def __init__(self, *args, **kwargs):
        self.cls_field = kwargs.pop('cls_field', '_cls')
        super(GenericEmbeddedDocumentField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        cls = None
        if isinstance(self.cls_field, (str, unicode)):
            cls = value[self.cls_field]
        elif hasattr(self.cls_field, '__call__'):
            cls = self.cls_field(value)

        if cls is not None and isinstance(value, dict):
            value.update({
                '_cls': cls,
            })
        return super(GenericEmbeddedDocumentField, self).to_python(value)

    def to_mongo(self, document):
        if document is None:
            return None

        data = document.to_mongo()
        # if not '_cls' in data:
        #     data['_cls'] = document._class_name
        return data

models.py

from fields import GenericEmbeddedDocumentField

PAYMENT_MAPPING = {
    'credit card': 'CreditCardPayment',
    'cheque': 'ChequePayment',
}

class Payment(db.EmbeddedDocument):
    price = db.IntField()
    currency = db.StringField()
    payment_type = db.StringField(choices=['credit card', 'cheque'])

    meta = {
        'allow_inheritance': False,
        'abstract': True,
    }

class CreditCardPayment(Payment):
    meta = {
        'allow_inheritance': False,
    }

class ChequePayment(Payment):
    meta = {
        'allow_inheritance': False,
    }
        
class MyModel(db.Document):
    payment = GenericEmbeddedDocumentField(choices=[CreditCardPayment, ChequePayment], cls_field=lambda value: PAYMENT_MAPPING[value['payment_type']])

Excellent tutorial!! Thanks!