django: saving a query set to session

Question

I am trying to save query result obtained in one view to session, and retrieve it in another view, so I tried something like below:

def default (request):
    equipment_list = Equipment.objects.all()

    request.session['export_querset'] = equipment_list

However, this gives me

TypeError at /calbase/

<QuerySet [<Equipment: A>, <Equipment: B>, <Equipment: C>]> is not JSON serializable

I am wondering what does this mean exactly and how should I go about it? Or maybe there is alternative way of doing what I want besides using session?

Answer 1

'e4c5' raises a concern which is perfectly valid. From the limited code we can see, it makes no sense to put in the results of that query into the session. Unless ofcourse you have some other plans which we cant quite see here. I'll ignore this and assume you ABSOLUTELY MUST save the query results into the session.

With this assumption, you must understand that the queryset instance which Django is giving you is a python object. You can move this around WITHIN your Django application without any hassles. However, whenever you attempt to send such an entity over the wire into some other data store/application (in your case, saving it into the session, which involves sending this data over to your configured session store), it must be serializable to some format which:

1. your application knows how to serialize objects into
2. the data store at the other end knows how to de-serialize. In this case, the accepted format seems to be JSON. (this is optional, the JSON string can be stored directly)

The problem is, the queryset instance not only contains the rows returned from the table, it also contains a bunch of other attributes and meta attributes which come in handy to you when you use the Django ORM API. When you try to send the queryset instance over the wire to your session store, the system knows no better and tries to serialize all these attributes into JSON. This fails because there are attributes within the queryset that are not serializable into JSON.

As far as a solution is concerned, if you must save data into the session, as some people have suggested, simply performing objects.all().values() and saving it into your session may not always work. A simple case is when your table returns datetime objects. Datetime objects are by default, not JSON serializable.

So what should you do? What you need is some sort of serializer which accepts a queryset, and safely iterates over the returned rows converting each python native datatype into a JSON safe equivalent, and then returning that. In case of datetime.datetime objects, you would need to call obj.isoformat() to transform it into an ISO format datetime string.