You have to use get_object
as func, not as property:
class PostViewSets(viewsets.ModelViewSet):
queryset = Post.objects.all()
serializer_class = PostSerializer
def get_serializer_context(self):
context = super(PostViewSets, self).get_serializer_context()
author = self.get_object().author
author_posts = self.get_queryset().filter(author=author)
context.update({'author_posts': author_posts})
return context