Add ExcludeFieldsOnEditMixin for edit handlers, disabling specific fields when bound to an existing instance rather than creating a new one

wagtail/admin/edit_handlers.py

@@ -789,19 +789,38 @@ Page.base_form_class = WagtailAdminPageForm
 #Similarly, set up wagtailcore.Workflow to have edit handlers
 Workflow.panels = [
                     FieldPanel("name"),
-                    FieldPanel("active"),
                     InlinePanel("workflow_tasks", heading="Tasks"),
                     ]
 Task.panels = [
                     FieldPanel("name"),
-                    FieldPanel("active"),
                     ]
 GroupApprovalTask.panels = Task.panels + [FieldPanel('group')]
+GroupApprovalTask.exclude_on_edit = {'group'}
 
 Workflow.base_form_class = WagtailAdminModelForm
 Task.base_form_class = WagtailAdminModelForm
 
 
+class ExcludeFieldsOnEditMixin:
+    """A mixin for edit handlers, which disables fields listed in a model's 'exclude_on_edit' attribute when binding
+    to an existing instance - editing rather than creating"""
+
+    def bind_to(self, *args, **kwargs):
+        new = super(ExcludeFieldsOnEditMixin, self).bind_to(*args, **kwargs)
+        # when binding to an existing instance - ie editing - set those fields in the form to disabled
+        if new.form and new.instance and hasattr(new.model, 'exclude_on_edit'):
+            for field in new.model.exclude_on_edit:
+                try:
+                    new.form.fields[field].disabled = True
+                except KeyError:
+                    continue
+        return new
+
+
+class VaryOnEditObjectList(ExcludeFieldsOnEditMixin, ObjectList):
+    pass
+
+
 @cached_classmethod
 def get_simple_edit_handler(cls):
     """
@@ -810,7 +829,7 @@ def get_simple_edit_handler(cls):
     if hasattr(cls, 'edit_handler'):
         edit_handler = cls.edit_handler
     else:
-        edit_handler = ObjectList(cls.panels, base_form_class=cls.base_form_class)
+        edit_handler = VaryOnEditObjectList(cls.panels, base_form_class=cls.base_form_class)
     return edit_handler.bind_to(model=cls)