Added optimize queries chapter

docs/conf.py

@@ -34,7 +34,7 @@
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
-# This will track teh Django version against whihch this is written.
+# This will track the Django version against which this is written.
 version = '2.0'
 
 release = '2.0'

docs/index.rst

@@ -33,7 +33,7 @@ Calculated fields
    :maxdepth: 1
 
    calculated_fields
-   many_to_many
+   optimize_queries
    sorting_calculated_fields
    filtering_calculated_fields
 
@@ -82,6 +82,7 @@ Listview Page
    disable_pagination
    date_based_filtering
    boolean_fields
+   many_to_many
 
 Changeview Page
 ++++++++++++++++++++++++++++++++++++++++++
@@ -110,7 +111,6 @@ Misc
    add_model_twice
    override_save
    database_view
-   optimize_queries
    set_ordering
 
 

docs/optimize_queries.rst

@@ -1,2 +1,47 @@
 How to optimize queries in Django admin?
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+If you have a lot of calculated fields in your admin, you can be running multipel queries per object leading to your admin can becoming quite slow.
+To fix this you can override the :code:`get_queryset` method on model admin to annotate the calculated fields.
+
+Lets take the example of this `ModelAdmin` we have for `Origin`::
+
+    @admin.register(Origin)
+    class OriginAdmin(admin.ModelAdmin):
+        list_display = ("name", "hero_count", "villain_count")
+
+        def hero_count(self, obj):
+            return obj.hero_set.count()
+
+        def villain_count(self, obj):
+            return obj.villain_set.count()
+
+
+This add two extra queries per row in your listview page. To fix this you can override the :code:`get_queryset` to annotate the counted fields,
+and then use the annotated fields in your ModelAdmin methods.
+
+With the changes, your ModelAdmin field looks like this::
+
+
+    @admin.register(Origin)
+    class OriginAdmin(admin.ModelAdmin):
+        list_display = ("name", "hero_count", "villain_count")
+
+        def get_queryset(self, request):
+            queryset = super().get_queryset(request)
+            queryset = queryset.annotate(
+                _hero_count=Count("hero", distinct=True),
+                _villain_count=Count("villain", distinct=True),
+            )
+            return queryset
+
+        def hero_count(self, obj):
+            return obj._hero_count
+
+        def villain_count(self, obj):
+            return obj._villain_count
+
+There are no per object extra queries. Your admin continues to look like it did before the :code:`annotate` call.
+
+.. image:: calculated_field.png
+

heroes_and_monsters/entities/admin.py

@@ -1,4 +1,5 @@
 from django.contrib import admin
+from django.db.models import Count
 
 from .models import Hero, Villain, Category, Origin
 
@@ -19,9 +20,16 @@ class CategoryAdmin(admin.ModelAdmin):
 class OriginAdmin(admin.ModelAdmin):
     list_display = ("name", "hero_count", "villain_count")
 
+    def get_queryset(self, request):
+        queryset = super().get_queryset(request)
+        queryset = queryset.annotate(
+            _hero_count=Count("hero", distinct=True),
+            _villain_count=Count("villain", distinct=True),
+        )
+        return queryset
 
     def hero_count(self, obj):
-        return obj.hero_set.count()
+        return obj._hero_count
 
     def villain_count(self, obj):
-        return obj.villain_set.count()
+        return obj._villain_count