I know its working, other wise you will not commit it. But I don't think its the right way of doing it. 

The problem is, `DeliveryWindow` is a Django Model class, and it doesn't have `order_list` as its property. You are adding a property `order_list` to a `DeliveryWindow` instance. This will results in inconsistent state between the in-memory `delivery_window` object and the one in db. We want to avoid this kinda of behavior. 

To summarize, we don't want to manipulate a `Model` object unless we are going to save the changes to the db.