views: Prefetch 'Patch.check_set'

The checks for each Patch are queried each time patches are listed.
This causes a deluge of queries (around one per patch). Mitigate this
by instead prefetching this related attribute. This reduces the number
of queries for a list of 100 patches from an average of 119 to 20.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>

Author: stephenfin

patchwork/views/__init__.py

@@ -263,6 +263,9 @@ def generic_list(request, project, view,
     # rendering the list template
     patches = patches.select_related('state', 'submitter', 'delegate')
 
+    # we also need checks
+    patches = patches.prefetch_related('check_set')
+
     paginator = Paginator(request, patches)
 
     context.update({