When working with RetrieveAPIView queryset must be defined and usually it is defined as queryset = <Model>.objects.all()

Why does retrieving a single instance require loading all the objects?

CodePudding user response：

You don't load all the objects. The queryset is not evaluated. The RetrieveAPIView ^[classy-drf] will use the queryset as a base to make queries. Indeed, in the .retrieve(…) method, it will obtain the object:

def retrieve(self, request, *args, **kwargs):
    instance = self.get_object()
    serializer = self.get_serializer(instance)
    return Response(serializer.data)

and the .get_object(…) method, it will fetch the single object:

def get_object(self):
    """
    Returns the object the view is displaying.

    You may want to override this if you need to provide non-standard
    queryset lookups.  Eg if objects are referenced using multiple
    keyword arguments in the url conf.
    """
    queryset = self.filter_queryset(self.get_queryset())

    # Perform the lookup filtering.
    lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field

    assert lookup_url_kwarg in self.kwargs, (
        'Expected view %s to be called with a URL keyword argument '
        'named "%s". Fix your URL conf, or set the `.lookup_field` '
        'attribute on the view correctly.' %
        (self.__class__.__name__, lookup_url_kwarg)
    )

    filter_kwargs = {self.lookup_field: self.kwargs[lookup_url_kwarg]}
    obj = get_object_or_404(queryset, **filter_kwargs)

    # May raise a permission denied
    self.check_object_permissions(self.request, obj)

    return obj

It will thus for example boil down to get_object_or_404(MyModel.objects.all(), pk=42), and this will make a query to fetch the single record with primary key 42. QuerySets are lazy: the MyModel.objects.all() is not evaluated unless the queryset is consumed, but here it thus is used to construct a new queryset that will eventually fetch a single record.