Prevent cascade deletion of pages when a ContentType is deleted

Fixes #2024

CHANGELOG.txt

@@ -7,6 +7,7 @@ Changelog
  * The `Document` model can now be overridden using the new `WAGTAILDOCS_DOCUMENT_MODEL` setting (Alex Gleason)
  * Fix: Custom page managers no longer raise an error when used on an abstract model
  * Fix: Wagtail's migrations are now all reversible (benjaoming)
+ * Fix: Deleting a page content type now preserves existing pages as basic Page instances, to prevent tree corruption
 
 1.3.1 (05.01.2016)
 ~~~~~~~~~~~~~~~~~~

docs/releases/1.4.rst

@@ -24,6 +24,7 @@ Bug fixes
 
  * Custom page managers no longer raise an error when used on an abstract model
  * Wagtail's migrations are now all reversible (benjaoming)
+ * Deleting a page content type now preserves existing pages as basic Page instances, to prevent tree corruption
 
 
 Upgrade considerations

wagtail/wagtailcore/migrations/0024_alter_page_content_type_on_delete_behaviour.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9 on 2015-12-22 09:34
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import wagtail.wagtailcore.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0023_alter_page_revision_on_delete_behaviour'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='page',
+            name='content_type',
+            field=models.ForeignKey(on_delete=models.SET(wagtail.wagtailcore.models.get_default_page_content_type), related_name='pages', to='contenttypes.ContentType', verbose_name='content type'),
+        ),
+    ]

wagtail/wagtailcore/models.py

@@ -205,6 +205,14 @@ def get_page_models():
     return PAGE_MODEL_CLASSES
 
 
+def get_default_page_content_type():
+    """
+    Returns the content type to use as a default for pages whose content type
+    has been deleted.
+    """
+    return ContentType.objects.get_for_model(Page)
+
+
 def get_page_types():
     """
     DEPRECATED.
@@ -277,7 +285,12 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
     )
     # TODO: enforce uniqueness on slug field per parent (will have to be done at the Django
     # level rather than db, since there is no explicit parent relation in the db)
-    content_type = models.ForeignKey('contenttypes.ContentType', verbose_name=_('content type'), related_name='pages')
+    content_type = models.ForeignKey(
+        'contenttypes.ContentType',
+        verbose_name=_('content type'),
+        related_name='pages',
+        on_delete=models.SET(get_default_page_content_type)
+    )
     live = models.BooleanField(verbose_name=_('live'), default=True, editable=False)
     has_unpublished_changes = models.BooleanField(
         verbose_name=_('has unpublished changes'),
@@ -452,8 +465,6 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
                             for model in [cls] + list(cls._meta.get_parent_list())
                             for field in model._meta.parents.values() if field]
 
-        field_exceptions += ['content_type']
-
         for field in cls._meta.fields:
             if isinstance(field, models.ForeignKey) and field.name not in field_exceptions:
                 if field.rel.on_delete == models.CASCADE:

wagtail/wagtailcore/tests/test_page_model.py

@@ -1036,3 +1036,23 @@ class TestPageManager(TestCase):
         their custom Manager inherits from PageManager.
         """
         self.assertIs(type(MyCustomPage.objects), CustomManager)
+
+
+class TestIssue2024(TestCase):
+    """
+    This tests that deleting a content type can't delete any Page objects.
+    """
+    fixtures = ['test.json']
+
+    def test_delete_content_type(self):
+        event_index = Page.objects.get(url_path='/home/events/')
+
+        # Delete the content type
+        event_index_content_type = event_index.content_type
+        event_index_content_type.delete()
+
+        # Fetch the page again, it should still exist
+        event_index = Page.objects.get(url_path='/home/events/')
+
+        # Check that the content_type changed to Page
+        self.assertEqual(event_index.content_type, ContentType.objects.get_for_model(Page))