title: DRF serializer.save() not saving to database
body: <p>I have an api that will do a patch on a resource (MyUser). It validates ok and seems to save the object, however when querying the database the changes have not been saved.</p>
  
  <pre><code>class UserSignupView(generics.UpdateAPIView):
    serializer_class = MyUserSerializer
  
    def get_object(self, email):
      obj = MyUser.objects.get(email=email)
      self.check_object_permissions(self.request, obj)
      return obj
  
    def patch(self, request):
      print(request.user)
      user = self.get_object(request.user.email)
      print(user.street)
      serializer = MyUserSerializer(user, data=request.data, partial=True)
      serializer.is_valid(raise_exception=True)
      serializer.save()
      savedUser = MyUser.objects.get(email=request.user.email)
      print(savedUser.street)
      print(serializer.data)
      return Response(serializer.data)
  
  
  class MyUserSerializer(serializers.ModelSerializer):
    class Meta:
      model = MyUser
      fields = (
          'id', 'first_name', 'last_name', 'email', 'phone_number', 'street', 'locality', 'city',
          'county', 'postcode')
  </code></pre>
  
  <p>Looking at the print statements I get:<br>
  user@example.com<br>
  None<br>
  123 Fake Street<br>
  MyUser object</p>
  
  <p>It returns the correct serialised data which contains the changes but the database does not have the changes.  The database connection is ok as I can query it and make other reads/writes/etc.  It's pretty much the same as the UpdateModelMixin except I've had to override the get_object with a passed in parameter.</p>