Question

I am trying to make a view that allows a user to edit a model instance (an event in this case). Unfortunately, submitting this form creates a new instance (with a new id) and doesn’t even delete the old instance. I’m under the impression that the save method is supposed to update the instance in this case…

NOTE: EventForm is a ModelForm

I’ve tried using the force_update arg per https://docs.djangoproject.com/en/dev/ref/models/instances/#forcing-an-insert-or-update, but no dice. I’ve also tried just deleting the original event in the form.is_valid() block (by calling event.delete()) but….no dice.

I have a feeling that the commit=False is the issue? I’m not sure!

Thanks.

(Please ignore spacing problems in code snippet)

def edit_event(request, event_id):
 event = Event.objects.get(pk=event_id)  

 if request.method == 'POST':
     post_data = request.POST.copy()

     # here is some validation that can't be done in the ModelForm...

     #form = EventForm(post_data, request.FILES, instance=event)
     form = EventForm(post_data, request.FILES)

     if form.is_valid():

         edited_event = form.save(commit=False)

         edited_event.save(force_update=True) # doesn't work with or without force_update arg

         #form.save_m2m() # needed for ManyToMany relationship

         return HttpResponseRedirect('/events/view/%s' % edited_event.id)
  else:
      form = EventForm(instance=event)

return render_to_response('create_event.html', {'form': form,}, context_instance=RequestContext(request) )

UPDATE

I got rid of the M2M relationship on my model, so I am able to get rid of the form.save_m2m() line. That still doesn’t work.

I also tried not passing the instance when submitting the form, under the assumption that the correct fields will be pre-populated when the user submits (which is the case right now). This still isn’t working.

Am I missing an important detail when it comes to updating a model??

Answer 1

From what I can understand, the problem with your code is that you’re always creating a new instance before saving. Since you’ve already obtained the instance of the event in “event = Event.objects.get(pk=event_id)”, you could simply change this instance and save it, and I’m quite sure that would cause django to make an update.

I advise you to check out this link:
https://docs.djangoproject.com/en/dev/ref/models/instances/?from=olddocs#how-django-knows-to-update-vs-insert

It has a pretty simple explanation on the algorithm Django uses to distinguish between inserts and updates. If your object doesn’t have it’s primary key set (probably the “id” field that django creates automatically when you syncdb), then it’ll be treated as a new object. If it has that field set, then Django will try to find that particular record in the database, if it exists, it’ll be updated.

Hope the answer was clear enough.