Django rest update one field

Question

I just got started with Django and got stuck on something that I believe should be simple, but I don't know how to do.

I have a model like this one:

id = models.AutoField(primary_key=true)
...
amount = models.IntegerField()
...

Basically, the user will give an amount and the model needs to be updated with the current amount + the amount that the user inputs.

I use serializers to create new objects, but I don't really know how to use them to do this.

Maybe a good starting point is this question: https://stackoverflow.com/q/41110742/719247 — Paolo Stefan, May 02 '18 at 08:10
@PaoloStefan I saw that post, but that would not have in mind the current value stored in the model, which is something I need. — ragtime.sp, May 02 '18 at 08:20

Paolo Stefan · Accepted Answer · 2018-05-02T09:32:31.853

13

Let's assume the following:

your model is called MyModel
your serializer class is named MyModelSerializer
AmountPartialUpdateView is extending APIView

your partial update url is defined like this -that is, model id is passed in the pk URL variable and the amount to add is passed in the amount URL variable:

urlpatterns = patterns('',
    # ...
    url(r'^model/update-partial/(?P<pk>\d+)/(?P<amount>\d+)$', AmountPartialUpdateView.as_view(), name='amount_partial_update'),
    # ...
)

Then, you should implement the correct update logic in the AmountPartialUpdateView.patch() method. One way to accomplish this is:

from django.shortcuts import get_object_or_404
from rest_framework import Response


class AmountPartialUpdateView(APIView):

    def patch(self, request, pk, amount):
        # if no model exists by this PK, raise a 404 error
        model = get_object_or_404(MyModel, pk=pk)
        # this is the only field we want to update
        data = {"amount": model.amount + int(amount)}
        serializer = MyModelSerializer(model, data=data, partial=True)

        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data)
        # return a meaningful error response
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

This way, visiting the URL

model/update-partial/123/5

you will increase the amount of model id 123 by 5 units.

edited May 02 '18 at 09:32

answered May 02 '18 at 08:56

Paolo Stefan

10,112
5
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How should be the URL I need to send the request to: whatever/model/update-partial/pk=1&amount=10 ? I am getting an 404 error on every combination I tried, and I know there is an object with that pk – ragtime.sp May 02 '18 at 09:21
Right, it does something now. I get an error unsupported operand types for +: int and str. Both model.amount and amount are int, so I don't get why is it complaining about that. – ragtime.sp May 02 '18 at 09:31
No, it's right: the `amount` in the URL segment is passed as a stirng to the patch() function - upated the answer again. – Paolo Stefan May 02 '18 at 09:33
OK, now I just need to create the update method in the serializer then. Let me try. – ragtime.sp May 02 '18 at 09:43
`def update(self, instance, validated_data): instance.amount = validated_data['amount'] instance.save() return instance` Shouldn't this work? I am getting an error simply saying KeyError amount – ragtime.sp May 02 '18 at 09:59
It's not mandatory: if your MyModelSerializer extends rest_framework.serializers.ModelSerializer, then my code will suffice without needing to override the serializer's update() method. – Paolo Stefan May 02 '18 at 10:01
1

Yep, that made the trick. Thank you so much! You are a life saver! – ragtime.sp May 02 '18 at 10:05

score 4 · Answer 2 · answered Aug 21 '19 at 10:59

just use the partial=True as one of your serializer parameter and create an object for your filed that you want to update i.e i want to update the queue status

    data = {'queue_status': 1}
serializer_patient_queue = PatientQueueSaveSerializer(queue_item, data=data, partial=True)

Django rest update one field

2 Answers2