admin: make the domain field read-only after creation

fixes #3937

wcsinst/wcsinst/admin.py

@@ -24,8 +24,20 @@ class WcsInstanceAdmin(admin.ModelAdmin):
             {'fields': ('drupal', 'ezldap', 'strongbox', 'clicrdv', 'domino' )}
         ),
     )
+    readonly_fields = ('domain',)
     inlines = [VariablesInline, ApiSecretsInline]
     save_as = True
 
+    def get_prepopulated_fields(self, request, obj=None):
+        if obj:
+            return {}
+        return self.prepopulated_fields
+
+    def get_readonly_fields(self, request, obj=None):
+        readonly_fields = self.readonly_fields
+        if not obj:
+            return filter(lambda x: x != 'domain', readonly_fields)
+        return readonly_fields
+
 
 admin.site.register(WcsInstance, WcsInstanceAdmin)