class Log(Document):
    ref = GenericReferenceField()
    user = ReferenceField('User')
    log_type = StringField()    # film or book or ...
    action = StringField()    # insert or update or delete
    created_at = DateTimeField(default=datetime.datetime.now)

"id" : {
    "$ref" : "film",
    "$id" : ObjectId("52283ae84c283a5713020770")
}