Fixed #17535 -- Optimized list generic views.

When allow_empty is False, prevented the view from loading
the entire queryset in memory when pagination is enabled.

django/views/generic/dates.py

@@ -273,7 +273,7 @@ def get_dated_queryset(self, **lookup):
         if not allow_empty:
             # When pagination is enabled, it's better to do a cheap query
             # than to load the unpaginated queryset in memory.
-            is_empty = not bool(qs) if paginate_by is None else not qs.exists()
+            is_empty = len(qs) == 0 if paginate_by is None else not qs.exists()
             if is_empty:
                 raise Http404(_(u"No %(verbose_name_plural)s available") % {
                         'verbose_name_plural': force_unicode(qs.model._meta.verbose_name_plural)

django/views/generic/list.py

@@ -16,7 +16,7 @@ class MultipleObjectMixin(ContextMixin):
 
     def get_queryset(self):
         """
-        Get the list of items for this view. This must be an interable, and may
+        Get the list of items for this view. This must be an iterable, and may
         be a queryset (in which qs-specific behavior will be enabled).
         """
         if self.queryset is not None:
@@ -113,9 +113,19 @@ class BaseListView(MultipleObjectMixin, View):
     def get(self, request, *args, **kwargs):
         self.object_list = self.get_queryset()
         allow_empty = self.get_allow_empty()
-        if not allow_empty and len(self.object_list) == 0:
-            raise Http404(_(u"Empty list and '%(class_name)s.allow_empty' is False.")
-                          % {'class_name': self.__class__.__name__})
+
+        if not allow_empty:
+            # When pagination is enabled and object_list is a queryset,
+            # it's better to do a cheap query than to load the unpaginated
+            # queryset in memory.
+            if (self.get_paginate_by(self.object_list) is not None
+                and hasattr(self.object_list, 'exists')):
+                is_empty = not self.object_list.exists()
+            else:
+                is_empty = len(self.object_list) == 0
+            if is_empty:
+                raise Http404(_(u"Empty list and '%(class_name)s.allow_empty' is False.")
+                        % {'class_name': self.__class__.__name__})
         context = self.get_context_data(object_list=self.object_list)
         return self.render_to_response(context)
 

tests/regressiontests/generic_views/list.py

@@ -159,6 +159,16 @@ def test_duplicate_context_object_name(self):
     def test_missing_items(self):
         self.assertRaises(ImproperlyConfigured, self.client.get, '/list/authors/invalid/')
 
+    def test_paginated_list_view_does_not_load_entire_table(self):
+        # Regression test for #17535
+        self._make_authors(3)
+        # 1 query for authors
+        with self.assertNumQueries(1):
+            self.client.get('/list/authors/notempty/')
+        # same as above + 1 query to test if authors exist + 1 query for pagination
+        with self.assertNumQueries(3):
+            self.client.get('/list/authors/notempty/paginated/')
+
     def _make_authors(self, n):
         Author.objects.all().delete()
         for i in range(n):

tests/regressiontests/generic_views/urls.py

@@ -128,6 +128,8 @@
         views.AuthorList.as_view(paginate_by=30)),
     (r'^list/authors/notempty/$',
         views.AuthorList.as_view(allow_empty=False)),
+    (r'^list/authors/notempty/paginated/$',
+        views.AuthorList.as_view(allow_empty=False, paginate_by=2)),
     (r'^list/authors/template_name/$',
         views.AuthorList.as_view(template_name='generic_views/list.html')),
     (r'^list/authors/template_name_suffix/$',