Django, how to include pre-existing data in update form view - django

Question

I am unable to see pre-existing form data when updating. The forms work fine, after submitting the database is updated, but in order to submit the user must enter all form data (including data that will not be updated). While reentering, the previous data is not visible. Is there a way to display the current data of the model instance being updated in the form fields?
Forms:
UpdateSomethingForm(forms.ModelForm):
class Meta:
model = Something
fields = ['field1', 'field2', 'field3']
Views:
def update_something(request, object_pk):
form = UpdateSomethingForm()
context_dict = {}
try:
instance = Something.objects.get(pk=object_pk)
context_dict['instance'] = instance
except Something.DoesNotExist:
context_dict['instance'] = None
if request.method == 'POST':
form = UpdateSomethingForm(request.POST, instance=instance)
if form.is_valid():
form.save(commit=True)
return HttpResponseRedirect('/home')
else:
print(form.errors)
context_dict['form'] = form
return render(request, 'form.html', context=context_dict)
Html:
<form role="form" method="post">
{% csrf_token %}
{{ form|bootstrap }}
<div class="form-group">
<button type="submit" class="btn btn-primary">Submit</button>
</div>
</form>

Answer 1

You passed the instance argument on POST, but not on GET.
form = UpdateSomethingForm(instance=instance)
in full:
def update_something(request, object_pk):
try:
instance = Something.objects.get(pk=object_pk)
except Something.DoesNotExist:
instance = None
if request.method == 'POST':
form = UpdateSomethingForm(request.POST, instance=instance)
if form.is_valid():
form.save()
return HttpResponseRedirect('/home')
else:
form = UpdateSomethingForm(instance=instance)
context_dict = {'form': form, 'instance': instance}
return render(request, 'form.html', context_dict)

Answer 2

The main problem is that you construct an empty Form, even if the instance can be found. But you make the view rather "chaotic" in the first place.
Probably a more readable view is:
def update_something(request, object_pk):
context_dict = {}
try:
instance = Something.objects.get(pk=object_pk)
except Something.DoesNotExist:
instance = None
context_dict['instance'] = instance
if request.method == 'POST':
form = UpdateSomethingForm(request.POST, instance=instance)
if form.is_valid():
form.save(commit=True)
return redirect('view_name')
else:
form = UpdateSomethingForm(instance=instance)
context_dict['form'] = form
return render(request, 'form.html', context=context_dict)
Here we ensure that the instance variable is always defined, also in the case the except body is "firing".
Furthermore it is probably better to use a redirect(..) and pass the name of the view over an URL, since if you change the URL of that view, this will still work.