Django modelForm update certain fields - django

Question

I have a 'Farm' model and a corresponding ModelForm as follows:
class FarmForm(ModelForm):
class Meta:
model = Farm
fields = ['farm_name','address','farm_size', 'latitude', 'longitude']
I can save a new Farm object through my client app (it requires that I fill in all the fields mentioned in my ModelForm).
I want to have another view where in I can update an existing Farm where the user can perhaps insert/update only those fields he/she wants to change. I tried something like following by passing only one of the field values through Postman but it gives me Form_not_valid error:
#api_view(['POST'])
def updateFarm(request, farmId):
farm = Farm.objects.get(id=farmId)
form = FarmForm(instance=farm, data=request.POST)
if form.is_valid():
farm = form.save()
farm = Farm.objects.filter(id=farm.id)
serializer = FarmSerializer(farm, many=True)
return JSONResponse(serializer.data)
#return Response("Data saved")
else:
return Response("Form not valid, insert correct fields.")
How can I build my view that let's user update only those fields he thinks are relevant? My url: url(r'^farms/update/(?P<farmId>\d\d)/$', views.updateFarm),

Answer 1

You can generate a boolean hidden form field for every field in your model, that gets set when a field is modified. For example name input:
<input id="id_name" maxlength="100" name="name" type="text">
will be followed by a name__specified hidden input:
<input id="id_name__specified" name="name__specified" type="hidden">
You track changes to field name with some js (very easy with plain js or jquery) and update name__specified accordingly to true/false.
In order to do this automatically and be able to re-use it, you can abstract this in a base form class and keep your form simple:
class BaseForm(forms.ModelForm):
suffix = '__specified'
def __init__(self, **kwargs):
super(BaseForm, self).__init__(**kwargs)
fields = list(self.fields)
for f in fields:
# Set the field default value from the instance
self.fields[f].widget.attrs['default'] = getattr(self.instance, f)
# JS tracking field changes
js = """
document.getElementById("id_%s").value =
this.value != this.getAttribute("default");
""" % (f + self.suffix)
self.fields[f].widget.attrs['onchange'] = js
self.fields[f + self.suffix] = forms.BooleanField(
widget=forms.HiddenInput(),
required=False
)
def clean(self):
data = super(BaseForm, self).clean()
flags = [f for f in self.fields if self.suffix in f]
for x in flags:
specified = data.get(x, False)
if not specified:
field = x[:-len(self.suffix)]
# If not specified grab it's current value from the instance
data[field] = getattr(self.instance, field)
# If the form validation complains that it's missing
# clear the error since we are not changing it's value
if field in self.errors:
del self.errors[field]
return data
So your modified form:
class FarmForm(BaseForm):
class Meta:
model = Farm
fields = ['farm_name','address','farm_size', 'latitude', 'longitude']
Note, you should pass the instance when instantiating a form in your GET function or simply inherit your view from UpdateView so that will be handled automatically:
class MyView(UpdateView):
template_name = 'my_template.html'
form_class = FarmForm
queryset = Farm.objects.all()
Now you can do partial updates!