If you use a viewset with a queryset using prefetch_related, any update to those prefetched many to many fields is not reflected in the response to the client, although the changes have been saved to the database.

See below code for very simple example:

from django.conf.urls import patterns, include, url
from django.contrib import admin
from django.contrib.auth.models import User
from rest_framework import serializers, viewsets, routers


class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('id', 'username', 'email', 'groups')


class UserViewSet(viewsets.ModelViewSet):
    queryset = User.objects.all().prefetch_related('groups')
    serializer_class = UserSerializer


router = routers.DefaultRouter()
router.register(r'users', UserViewSet)

urlpatterns = patterns('',
    url(r'^', include(router.urls)),
    url(r'^api-auth/', include('rest_framework.urls',
                               namespace='rest_framework')),
    url(r'^admin/', include(admin.site.urls)),
)

With the above code, assume there is an existing user with 2 groups assigned:

{
    "id": 1, 
    "username": "chris", 
    "email": "chris@example.com", 
    "groups": [
        1,
        2
    ]
}

Submitting a PUT to /users/1/ changing groups to [1], the response will still show groups as [1, 2], but if you then did a GET request for /users/1/ you would see the reflected changes.

You can see this behaviour using the browseable api also.

Removing prefetch_related from the queryset restores normal behaviour and edits are again reflected in the response.

This example is very trivial, but if people are using prefetch_related to try and optimise their database queries they may encounter this issue.