Make less DB queries (particularly in the Forum)

counter/models.py

@@ -29,6 +29,7 @@ from django.conf import settings
 from django.core.urlresolvers import reverse
 from django.forms import ValidationError
 from django.contrib.sites.shortcuts import get_current_site
+from django.utils.functional import cached_property
 
 from datetime import timedelta, date
 import random
@@ -215,6 +216,10 @@ class Counter(models.Model):
             p.end = p.activity
             p.save()
 
+    @cached_property
+    def barmen_list(self):
+        return self.get_barmen_list()
+
     def get_barmen_list(self):
         """
         Returns the barman list as list of User
@@ -246,7 +251,7 @@ class Counter(models.Model):
             p.save() # Update activity
 
     def is_open(self):
-        return len(self.get_barmen_list()) > 0
+        return len(self.barmen_list) > 0
 
     def is_inactive(self):
         """

forum/models.py

@@ -136,10 +136,19 @@ class Forum(models.Model):
     def last_message(self):
         return self.get_last_message()
 
+    forum_list = {} # Class variable used for cache purpose
     def get_last_message(self):
         last_msg = None
-        for m in ForumMessage.objects.select_related('topic__forum', 'author').order_by('-id'):
+        for m in ForumMessage.objects.select_related('topic__forum').order_by('-id'):
+            if m.topic.forum.id in Forum.forum_list.keys(): # The object is already in Python's memory,
+                                                            # so there's no need to query it again
+                forum = Forum.forum_list[m.topic.forum.id]
+            else: # Query the forum object and store it in the class variable for further use.
+                  # Keeping the same object allows the @cached_property to work properly.
+# This trick divided by 4 the number of DB queries in the main forum page, and about the same on many other forum pages.
+# This also divided by 4 the amount of CPU usage for thoses pages, according to Django Debug Toolbar.
                 forum = m.topic.forum
+                Forum.forum_list[forum.id] = forum
             if self in (forum.parent_list + [forum]) and not m.deleted:
                 return m