Added chapter on filtering on calculated fields

docs/filtering_calculated_fields.rst

@@ -1,2 +1,45 @@
 How to enable filtering on calculated fields?
 ===========================================================
+
+
+You have a :code:`Hero` admin which looks like this::
+
+    @admin.register(Hero)
+    class HeroAdmin(admin.ModelAdmin):
+        list_display = ("name", "is_immortal", "category", "origin", "is_very_benevolent")
+        list_filter = ("is_immortal", "category", "origin",)
+
+        def is_very_benevolent(self, obj):
+            return obj.benevolence_factor > 75
+
+It has one calculated field :code:`is_very_benevolent`, and your admin looks like this
+
+.. image:: no_filtering.png
+
+You have added filtering on the fields which come from the models, but you also want to add filtering on the calculated field. To do this, you will need to subclass
+:code:`SimpleListFilter` like this::
+
+
+    class IsVeryBenevolentFilter(admin.SimpleListFilter):
+        title = 'is_very_benevolent'
+        parameter_name = 'is_very_benevolent'
+
+        def lookups(self, request, model_admin):
+            return (
+                ('Yes', 'Yes'),
+                ('No', 'No'),
+            )
+
+        def queryset(self, request, queryset):
+            value = self.value()
+            if value == 'Yes':
+                return queryset.filter(benevolence_factor__gt=75)
+            elif value == 'No':
+                return queryset.exclude(benevolence_factor__gt=75)
+            return queryset
+
+And then change your :code:`list_filter` to :code:`list_filter = ("is_immortal", "category", "origin", IsVeryBenevolentFilter)`.
+
+With this you can filter on the calcualted field, and your admin looks like this:
+
+.. image:: filter_calculated_fixed.png

heroes_and_monsters/entities/admin.py

@@ -6,9 +6,32 @@
 admin.site.register(Villain)
 
 
+class IsVeryBenevolentFilter(admin.SimpleListFilter):
+    title = 'is_very_benevolent'
+    parameter_name = 'is_very_benevolent'
+
+    def lookups(self, request, model_admin):
+        return (
+            ('Yes', 'Yes'),
+            ('No', 'No'),
+        )
+
+    def queryset(self, request, queryset):
+        value = self.value()
+        if value == 'Yes':
+            return queryset.filter(benevolence_factor__gt=75)
+        elif value == 'No':
+            return queryset.exclude(benevolence_factor__gt=75)
+        return queryset
+
+
 @admin.register(Hero)
 class HeroAdmin(admin.ModelAdmin):
-    list_display = ("name", "gender", "is_immortal", "category", "origin")
+    list_display = ("name", "is_immortal", "category", "origin", "is_very_benevolent")
+    list_filter = ("is_immortal", "category", "origin", IsVeryBenevolentFilter)
+
+    def is_very_benevolent(self, obj):
+        return obj.benevolence_factor > 75
 
 
 @admin.register(Category)