Added add model twice chapter

docs/add_model_twice.rst

@@ -1,2 +1,36 @@
 How to add a model twice to Django admin?
-++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++++++++++++++++++++++
+
+
+You need to add the :code:`Hero` model twice to the admin, one as a regular admin area, and one as read only admin. (SOme user will potentially see only the read only admin.)
+
+If you have try to register the same model twice::
+
+    admin.site.register(Hero)
+    admin.site.register(Hero)
+
+you will get an error like this::
+
+    raise AlreadyRegistered('The model %s is already registered' % model.__name__)
+
+THe solution is to sublass the :code:`Hero` model as a ProxyModel.::
+
+    # In models.py
+    class HeroProxy(Hero):
+
+        class Meta:
+            proxy = True
+
+    ...
+    # In admin.py
+    @admin.register(Hero)
+    class HeroAdmin(admin.ModelAdmin, ExportCsvMixin):
+        list_display = ("name", "is_immortal", "category", "origin", "is_very_benevolent")
+        ....
+
+
+    @admin.register(HeroProxy)
+    class HeroProxyAdmin(admin.ModelAdmin):
+        readonly_fields = ("name", "is_immortal", "category", "origin",
+            ...)
+

heroes_and_monsters/entities/admin.py

@@ -1,7 +1,7 @@
 from django.contrib import admin
 from django.db.models import Count
 
-from .models import Hero, Villain, Category, Origin
+from .models import Hero, Villain, Category, Origin, HeroProxy
 
 import csv
 import sys
@@ -85,6 +85,13 @@ def is_very_benevolent(self, obj):
     is_very_benevolent.boolean = True
 
 
+@admin.register(HeroProxy)
+class HeroProxyAdmin(admin.ModelAdmin):
+    list_display = ("name", "is_immortal", "category", "origin",)
+    readonly_fields = ("name", "is_immortal", "category", "origin",)
+
+
+
 @admin.register(Villain)
 class VillainAdmin(admin.ModelAdmin, ExportCsvMixin):
     list_display = ("name", "category", "origin")

heroes_and_monsters/entities/models.py

@@ -79,6 +79,11 @@ class Meta:
     )
 
 
+class HeroProxy(Hero):
+
+    class Meta:
+        proxy = True
+
 class Villain(Entity):
     is_immortal = models.BooleanField(default=False)