class Person(Document):
    name = StringField()
age = StringField()
meta = {'allow_inheritance': True}
for p in Person.objects:
    p.age = '%s' % p.age
p.save()
rozza = Person.objects.first()
self.assertEqual('35.0', rozza.age)
if __name__ == '__main__':
    unittest.main()
Migrations in mongoengine: InvalidId