Added clauclted fields chapter

docs/calculated_fields.rst

@@ -1,2 +1,55 @@
 How to show calculated fields on listview page?
 ===========================================================
+
+You have an admin for the :code:`Origin` model like this::
+
+    @admin.register(Origin)
+    class OriginAdmin(admin.ModelAdmin):
+        list_display = ("name",)
+
+
+Apart from the name, we also want to show the number of heroes and number of villains for each origin, which is not a DB field on :code:`Origin`.
+You can do this in two ways.
+
+
+Adding a method to the model
+===========================================================
+
+You can add two methods to your :code:`Origin` model like this::
+
+
+    def hero_count(self,):
+        return self.hero_set.count()
+
+    def villain_count(self):
+        return self.villain_set.count()
+
+And change :code"`list_display` to :code:`list_display = ("name", "hero_count", "villain_count")`.
+
+
+Adding a method to the ModelAdmin
+===========================================================
+
+If you don't want to add method to the model, you can do instead add the method to the ModelAdmin. ::
+
+
+
+    def hero_count(self, obj):
+        return obj.hero_set.count()
+
+    def villain_count(self, obj):
+        return obj.villain_set.count()
+
+
+The :code:`list_display`, as earlier, changes to :code:`list_display = ("name", "hero_count", "villain_count")`.
+
+Performance considerations for calculated_fields
+===========================================================
+
+With either of the above approaches, you would be running two exta queries per object (One per caluclated field). You can find how to optimize this in
+:doc:`optimize_queries`.
+
+
+With any of these changes your admin looks like this:
+
+.. image:: calculated_field.png

docs/conf.py

@@ -34,10 +34,10 @@
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
-# The short X.Y version.
-version = '0.1'
-# The full version, including alpha/beta/rc tags.
-release = '0.1'
+# This will track teh Django version against whihch this is written.
+version = '2.0'
+
+release = '2.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.

docs/index.rst

@@ -25,7 +25,6 @@ Text and Design
    remove_default
    logo
    override_default_templates
-   set_ordering
 
 Calculated fields
 +++++++++++++++++++++

heroes_and_monsters/entities/admin.py

@@ -2,7 +2,26 @@
 
 from .models import Hero, Villain, Category, Origin
 
-admin.site.register(Hero)
 admin.site.register(Villain)
-admin.site.register(Category)
-admin.site.register(Origin)
+
+
+@admin.register(Hero)
+class HeroAdmin(admin.ModelAdmin):
+    list_display = ("name", "gender", "is_immortal", "category", "origin")
+
+
+@admin.register(Category)
+class CategoryAdmin(admin.ModelAdmin):
+    list_display = ("name",)
+
+
+@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()