Added chapter from custom button

docs/custom_button.rst

@@ -1,2 +1,43 @@
 How to add a custom button to Django change view page?
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+:code:`Villain` has a field called :code:`is_unique`::
+
+    class Villain(Entity):
+        ...
+        is_unique = models.BooleanField(default=True)
+
+
+You want to add a button on Villain change form page called "Make Unique", which make this Villain unique.
+Any other villain with the same name should be deleted.
+
+You start by extending the :code:`change_form` to add a new button.::
+
+    {% extends 'admin/change_form.html' %}
+
+    {% block submit_buttons_bottom %}
+        {{ block.super }}
+        <div class="submit-row">
+                <input type="submit" value="Make Unique" name="_make-unique">
+        </div>
+    {% endblock %}
+
+Then you can override :code:`response_change` and connect your template to the :code:`VillainAdmin`.::
+
+    @admin.register(Villain)
+    class VillainAdmin(admin.ModelAdmin, ExportCsvMixin):
+        ...
+        change_form_template = "entities/villain_changeform.html"
+
+
+        def response_change(self, request, obj):
+            if "_make-unique" in request.POST:
+                matching_names_except_this = self.get_queryset(request).filter(name=obj.name).exclude(pk=obj.id)
+                matching_names_except_this.delete()
+                obj.is_umique = True
+                obj.save()
+                self.message_user(request, "This villain is now unique")
+                return HttpResponseRedirect(".")
+            super().response_change()
+
+.. image:: custom_button.png

docs/index.rst

@@ -1,4 +1,4 @@
-Django Admin Cookbook
+.Django Admin Cookbook
 ===========================================================
 
 Django Admin Cookbook - How to do things with Django admin.

heroes_and_monsters/entities/admin.py

@@ -191,6 +191,18 @@ class VillainAdmin(admin.ModelAdmin, ExportCsvMixin):
     actions = ["export_as_csv"]
 
     readonly_fields = ["added_on"]
+    change_form_template = "entities/villain_changeform.html"
+
+
+    def response_change(self, request, obj):
+        if "_make-unique" in request.POST:
+            matching_names_except_this = self.get_queryset(request).filter(name=obj.name).exclude(pk=obj.id)
+            matching_names_except_this.delete()
+            obj.is_umique = True
+            obj.save()
+            self.message_user(request, "This villain is now unique")
+            return HttpResponseRedirect(".")
+        super().response_change()
 
 
 class VillainInline(admin.StackedInline):