python – Django Rest Framework partial update-ThrowExceptions

Exception or error:

I’m trying to implement partial_update with Django Rest Framework but I need some clarification because I’m stuck.

  1. Why do we need to specify partial=True?
    In my understanding, we could easily update Demo object inside of partial_update method. What is the purpose of this?

  2. What is inside of serialized variable?
    What is inside of serialized variable in partial_update method? Is that a Demo object? What function is called behind the scenes?

  3. How would one finish the implementation here?

Viewset

class DemoViewSet(viewsets.ModelViewSet):
    serializer_class = DemoSerializer

    def partial_update(self, request, pk=None):
        serialized = DemoSerializer(request.user, data=request.data, partial=True)
        return Response(status=status.HTTP_202_ACCEPTED)

Serializer

class DemoSerializer(serializers.ModelSerializer):
    class Meta:
        model = Demo
        fields = '__all__'

    def update(self, instance, validated_data):
        print 'this - here'
        demo = Demo.objects.get(pk=instance.id)
        Demo.objects.filter(pk=instance.id)\
                           .update(**validated_data)
        return demo
How to solve:

I have the same questions as yours before, but when I dig into the source code of rest_framework, I got the following findings, hope it would help:

For question 1)

This question is related to HTTP verbs.

PUT: The PUT method replaces all current representations of the target resource with the request payload.

PATCH: The PATCH method is used to apply partial modifications to a resource.

Generally speaking, partial is used to check whether the fields in the model is needed to do field validation when client submitting data to the view.

For example, we have a Book model like this, pls note both of the name and author_name fields are mandatory (not null & not blank).

class Book(models.Model):
    name = models.CharField('name of the book', max_length=100)
    author_name = models.CharField('the name of the author', max_length=50)

# Create a new instance for testing
Book.objects.create(name='Python in a nut shell', author_name='Alex Martelli')

For some scenarios, We may only need to update part of the fields in the model, e.g., we only need to update name field in the Book. So for this case, client will only submit the name field with new value to the view. The data submit from the client may look like this:

{"pk": 1, name: "PYTHON IN A NUT SHELL"}

But you may have notice that our model definition does not allow author_name to be blank. So we have to use partial_update instead of update. So the rest framework will not perform field validation check for the fields which is missing in the request data.

For testing purpose, you can create two views for both update and partial_update, and you will get more understanding what I just said.

Example:

views.py

from rest_framework.generics import GenericAPIView
from rest_framework.mixins import UpdateModelMixin
from rest_framework.viewsets import ModelViewSet
from rest_framework import serializers


class BookSerializer(serializers.ModelSerializer):
    class Meta:
        model = Book


class BookUpdateView(GenericAPIView, UpdateModelMixin):
    '''
    Book update API, need to submit both `name` and `author_name` fields
    At the same time, or django will prevent to do update for field missing
    '''
    queryset = Book.objects.all()
    serializer_class = BookSerializer

    def put(self, request, *args, **kwargs):
        return self.update(request, *args, **kwargs)

class BookPartialUpdateView(GenericAPIView, UpdateModelMixin):
    '''
    You just need to provide the field which is to be modified.
    '''
    queryset = Book.objects.all()
    serializer_class = BookSerializer

    def put(self, request, *args, **kwargs):
        return self.partial_update(request, *args, **kwargs)

urls.py

urlpatterns = patterns('',
    url(r'^book/update/(?P<pk>\d+)/$', BookUpdateView.as_view(), name='book_update'),
    url(r'^book/update-partial/(?P<pk>\d+)/$', BookPartialUpdateView.as_view(), name='book_partial_update'),
)

Data to submit

{"pk": 1, name: "PYTHON IN A NUT SHELL"}

When you submit the above json to the /book/update/1/, you will got the following error with HTTP_STATUS_CODE=400:

{
  "author_name": [
    "This field is required."
  ]
}

But when you submit the above json to /book/update-partial/1/, you will got HTTP_STATUS_CODE=200 with following response,

{
  "id": 1,
  "name": "PYTHON IN A NUT SHELL",
  "author_name": "Alex Martelli"
}

For question 2)

serialized is a object wrapping the model instance as a serialisable object. and you can use this serialized to generate a plain JSON string with serialized.data .

For question 3)

I think you can answer yourself when you have read the answer above, and you should have known when to use update and when to used partial_update.

If you still have any question, feel free to ask. I just read part of the source odes of rest framework, and may have not understand very deeply for some terms, and please point it out when it is wrong…

Answer:

For partial updatePATCH http method

For full updatePUT http method

When doing an update with DRF, you are supposed to send request data that includes values for all (required) fields. This is at least the case when the request is via the PUT http method. From what I understand, you want to update one or at least not all model instance fields. In this case make a request with the PATCH http method. Django rest framework (DRF) will take care of it out of the box.

Example (with token auth):

curl -i -X PATCH -d '{"name":"my favorite banana"}' -H "Content-Type: application/json" -H 'Authorization: Token <some token>'  http://localhost:8000/bananas/

Answer:

Just a quick note as it seems that nobody has already pointed this out:

serialized = DemoSerializer(request.user, data=request.data, partial=True)

The first argument of DemoSerializer should be a Demo instance, not a user (at least if you use DRF 3.6.2 like me).

I don’t know what you are trying to do, but this is a working example:

def partial_update(self, request, *args, **kwargs):
    response_with_updated_instance = super(DemoViewSet, self).partial_update(request, *args, **kwargs)
    Demo.objects.my_func(request.user, self.get_object())
    return response_with_updated_instance

I do the partial update and then I do other things calling my_func and passing the current user and the demo instance already updated.

Hope this helps.

Answer:

I had an issue where my multi-attribute/field validation in a rest_framework serializer was working with a POST /resources/ request but failing with a PATCH /resources/ request. It failed in the PATCH case because it was only looking for values in the supplied attrs dict and not falling back to values in self.instance. Adding a method get_attr_or_default to do that fallback seems to have worked:

class EmailSerializer(serializers.ModelSerializer):

    def get_attr_or_default(self, attr, attrs, default=''):
        """Return the value of key ``attr`` in the dict ``attrs``; if that is
        not present, return the value of the attribute ``attr`` in
        ``self.instance``; otherwise return ``default``.
        """
        return attrs.get(attr, getattr(self.instance, attr, ''))

    def validate(self, attrs):
        """Ensure that either a) there is a body or b) there is a valid template
        reference and template context.
        """
        existing_body = self.get_attr_or_default('body', attrs).strip()
        if existing_body:
            return attrs
        template = self.get_attr_or_default('template', attrs)
        templatecontext = self.get_attr_or_default('templatecontext', attrs)
        if template and templatecontext:
            try:
                render_template(template.data, templatecontext)
                return attrs
            except TemplateRendererException as err:
                raise serializers.ValidationError(str(err))
        raise serializers.ValidationError(NO_BODY_OR_TEMPLATE_ERROR_MSG)

Answer:

Just override init method of your serializer like that:

def __init__(self, *args, **kwargs):
    kwargs['partial'] = True
    super(DemoSerializer, self).__init__(*args, **kwargs)

Answer:

You forgot serializer.save()

You can finish it the following way . . .

class DemoViewSet(viewsets.ModelViewSet):
    serializer_class = DemoSerializer

    def partial_update(self, request, pk=None):
        serializer = DemoSerializer(request.user, data=request.data, partial=True)
        serializer.save()
        serializer.is_valid(raise_exception=True)
        return Response(serializer.data)

Also, you shouldn’t need to override the update method in the serializer.

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