How to store django objects as session variables ( object is not JSON serializable)?

In a session, I'd just store the object primary key:

request.session['card'] = card.id

and when loading the card from the session, obtain the card again with:

try:
    card = Card.objects.get(id=request.session['card'])
except (KeyError, Card.DoesNotExist):
    card = None

which will set card to None if there isn't a card entry in the session or the specific card doesn't exist.

By default, session data is serialised to JSON. You could also provide your own serializer, which knows how to store the card.id value or some other representation and, on deserialization, produce your Card instance again.

There are two simple ways to do this.

• If each object belongs to a single session at the same time, store session id as a model field, and update models.
• If an object can belong to multiple sessions at the same time, store object.id as a session variable.

Unfortunately the suggested answer does not work if the object is not a database object but some other kind of object - say, datetime or an object class Foo(object): pass that isn't a database model object.

Sure, if the object happen to have some id field you can store the id field in the database and look up the value from there but in general it may not have such a simple value and the only way is to convert the data to string in such a way that you can read that string and reconstruct the object based on the information in the string.

In the case of a datetime object this is made more complicated by the fact that while a naive datetime object can print out format %Z by simply not printing anything, the strptime object cannot read format %Z if there is nothing, it will choke unless there is a valid timezone specification there - so if you have a datetime object that may or may not contain a tzinfo field you really have to do strptime twice once with %Z and then if it chokes without the %Z. This is silly. It is made even sillier by the fact that datetime objects have a fromtimestamp function but no totimestamp function that uniformly produces a timestamp that fromtimestamp will read. If there is a format code that produces timestamp number I haven't found one and again, strftime/strptime suffer from the fact that they are not symmetric as described above.