MongoEngine not saving embedded doc second time around - mongodb

Question

When I use MongoEngine to add an embedded doc to a doc, it works the first time when the list is empty but fails with subsequent tries saying: mongoengine.errors.OperationError: Could not save document (Cannot update 'sensorlist.1.alert_list.0._cls' and 'sensorlist.1.alert_list' at the same time)
The following test code demonstrates the issue: If you run it once you will see a new collection/document in foo that has an S2 embedded Sensor with qty 2 embedded Alerts. If you run it again it blows up - Any ideas?
Thx Bill
import mongoengine as ME
ME.connect('foo')
class Sensor(ME.EmbeddedDocument):
name = ME.StringField()
alert_list = ME.ListField()
class Alert(ME.EmbeddedDocument):
name = ME.StringField(default = 'new alert')
class SiteConfig(ME.Document):
siteid = ME.StringField()
sensorlist = ME.ListField(ME.EmbeddedDocumentField(Sensor))
if not SiteConfig.objects(siteid = '123456'):
newsite = SiteConfig(siteid = '123456')
newsite.save()
print("saved new site")
site = SiteConfig.objects(siteid = '123456').first()
newsensor = Sensor(name='S1')
site.sensorlist.append(newsensor)
site.save()
print("added sensor S1")
newsensor = Sensor(name='S2')
site.sensorlist.append(newsensor)
site.save()
print("added sensor S2")
for sensor in site.sensorlist:
if sensor.name =='S2':
alert = Alert()
sensor.alert_list.append(alert)
site.save()
print('added first alert to S2')
for sensor in site.sensorlist:
if sensor.name =='S2':
alert = Alert()
sensor.alert_list.append(alert)
site.save()
print('added second alert to S1')

Answer 1

I solved this by reproducing the code in MongoAlchemy which is very similar. That also failed but actually gave me useful exception info that pointed me to the problem which was that my Sensor class Alert_list definition needed to specify the class it contains.
So the fix is to define Sensor like this:
class Sensor(ME.DynamicEmbeddedDocument):
name = ME.StringField()
alert_list = ME.ListField(ME.EmbeddedDocumentField(Alert))
After all that pain over such a small issue I will probably stick with MongoAlchemy!