strange issue testing guide

[Luca Paterlini]

from mongoengine import  *
from datetime import datetime
connect('mydb')
class session(Document):
    created = DateTimeField(default=datetime.now)
    meta = {
        'indexes': [
            {'fields': ['created'], 'expireAfterSeconds': 1}
        ]
    }
session().save()

I've copied the class from

2.3.5.4. Time To Live indexes

so i don't uderstood why i doesn't work (i've the last mongodb 3.x)

Many tanks to who will answer