Pfthb

I have a model storing, among other things, a timezone datetime object. I am reaching out with an Ajax call from the website to one of my views, which does the following call:

def uploads(request):

 user = User.objects.get(username=request.user.get_username())
 cases = Case.objects.filter(user_id=user.pk).order_by('-uploaded_on')[:5]

 return JsonResponse(serializers.serialize('json', cases, fields=('col1', 'col2', 'col3', 'uploaded_on')), safe=False)

When a JSON response is received at the Front End, it gets parsed with JS and analyzed. What I would like to do here, is to modify the 'uploaded_on' column with user's timezone (which I can already access with user.timezone, currently displayed as a string e.g. 'Germany/Berlin', but can easily convert it to a necessary object with e.g. pytz package).

When I try to iterate through the QuerySet like below, the values received on the Front End do not change:

for case in cases:
 case.uploaded_on.astimezone(pytz.timezone(user.timezone))

This is probably related to the fact that QuerySets are lazy, like described in Django's documentation. Can anyone suggest how this can be done?

A bonus question as well: with Django's serializer, can I pass along more than one QuerySet in a single JSON response? When I try to pack a few in a list, I get a MultiValueDictKeyError:

return JsonResponse(serializers.serialize('json', [cases, cases2]), safe=False)

To me there would be two approaches:

1. 
   

   Annotate the cases queryset with a new column, something like uploaded_at_utz and do timezone manipulation at database level. For example you could use TruncSecond function to do something like the following:

   
   
   

   cases = Case.objects.filter(user_id=user.pk).annotate(uploaded_at_utz=TruncSecond('uploaded_on', tzinfo=pytz.timezone(user.timezone)).order_by('-uploaded_on')[:5]
   

   
   


2. 
   

   The other approach would be to do this at python level. For this I would go for a generator function, something like:

   
   
   

   def convert_to_timezone(cases, tzinfo):
    for case in cases:
    case.uploaded_on = case.uploaded_on.astimezone(tzinfo)
    yield case
   

   
   

Then, in you would pass convert_to_timezone(cases, pytz.timezone(user.timezone)) to your JsonResponse constructor.

You should also explore the use of timezone.override and/or timezone.localtime although I'm not familiar with them.

Regarding the date manipulation, datetime.astimezone() returns a new datetime rather than converting in-place. You'd need to assign the converted value back to case.uploaded_on.

for case in cases:
 case.uploaded_on = case.uploaded_on.astimezone(pytz.timezone(user.timezone))

Regarding whether you can pass along more than one queryset in a single JSON response, you can use itertools.chain to do that.

from itertools import chain

return JsonResponse(serializers.serialize('json', chain(cases, cases2)), safe=False)