OperationError: Could not save document (LEFT_SUBFIELD only supports Object: ancestors.0 not: 7)

Question

I have an Organization database in MongoDB. I am trying to save data in that database using mongoengine. I am using Djnago server. when i am creating the object then its working fine but after editing its giving some error.

class Organization(Document):
    username= StringField()
    ancestors = ListField(ReferenceField('Organization',dbref=False), default = list)
    parents = ListField(ReferenceField('Organization',dbref=False),default = list)
    descendants = ListField(ReferenceField('Organization',dbref=False), default = list)


obj1 = Organization(username = 'kousik')
obj1.save()
<Organization: Organization object> #obj1 created

obj2 = Organization(username = 'chowdhury',ancestors = [obj1],parents=[obj1])
obj2.save()
<Organization: Organization object> #obj2 created

obj3 = Organization(username = 'kchowdhury',ancestors = [obj1,obj2],parents=[obj2])
obj3.save()
<Organization: Organization object> #obj3 creaed

obj1.descendants = [obj2,obj3]
obj1.save()
<Organization: Organization object> #obj1 updated

obj2.descendants = [obj3]
obj2.save()

Traceback (most recent call last):
  File "<pyshell#43>", line 1, in <module>
    obj2.save()
  File "C:\Python27\lib\site-packages\mongoengine\document.py", line 267, in save
    raise OperationError(message % unicode(err))
OperationError: Could not save document (LEFT_SUBFIELD only supports Object: ancestors.0 not: 7)

Answer 1

This is mongoengine bug. I created issue for this: https://github.com/MongoEngine/mongoengine/issues/422.

Solutions for now:

1. Use full reload document before updating (just reload don't work, because leave referencies):

   obj1 = Organization.objects(username=obj1.username).first()
   obj1.descendants = [obj2, obj3]
   obj1.save()
   
   
   obj2 = Organization.objects(username=obj2.username).first()
   obj2.descendants = [obj3]
   obj2.save()
   

2. Use atomic updates instead save.

3. Use to_dbref for referencies:

   obj1 = Organization(username='kousik')
   print obj1.save()
   # <Organization: Organization object> #obj1 created
   
   obj2 = Organization(username='chowdhury', ancestors=[obj1.to_dbref()], parents=[obj1.to_dbref()])
   print obj2.save()
   # <Organization: Organization object> #obj2 created
   
   obj3 = Organization(username='kchowdhury', ancestors=[obj1.to_dbref(), obj2.to_dbref()], parents=[obj2.to_dbref()])
   print obj3.save()
   # <Organization: Organization object> #obj3 creaed
   
   obj1.descendants = [obj2.to_dbref(), obj3.to_dbref()]
   print obj1.save()
   # <Organization: Organization object> #obj1 updated
   
   obj2.descendants = [obj3.to_dbref()]
   print obj2.save()
   # <Organization: Organization object> #obj2 updated