Added uneditable_exiting chapter

docs/uneditable_existing.rst

@@ -1,2 +1,14 @@
 How to make a field editable while creating, but read only in existing objects?
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+YOu need to make the :code:`name` and :code:`category` read only once a :code:`Hero` is created. However duing the first write the fields needs to be editable.
+
+You can do this by overriding :code:`get_readonly_fields` method, like this::
+
+    def get_readonly_fields(self, request, obj=None):
+        if obj:
+            return ["name", "category"]
+        else:
+            return []
+
+:code:`obj` is None during the object creation, but set to the object being edited during an edit.

heroes_and_monsters/entities/admin.py

@@ -55,6 +55,12 @@ class HeroAdmin(admin.ModelAdmin, ExportCsvMixin):
 
     exclude = ['added_by',]
 
+    def get_readonly_fields(self, request, obj=None):
+        if obj:
+            return ["name", "category"]
+        else:
+            return []
+
     def save_model(self, request, obj, form, change):
         if not obj.pk:
             # Only set added_by during the first save.