Add migration to replace JSON null values with empty objects in log entries' data

CHANGELOG.txt

@@ -75,6 +75,7 @@ Changelog
  * Fix: Fix preview panel loading issues (Sage Abdullah)
  * Fix: Fix `search_promotions` `0004_copy_queries` migration for long-lived Wagtail instances (Sage Abdullah)
  * Fix: Guard against `TypeError` in `0088_fix_log_entry_json_timestamps` migration (Sage Abdullah)
+ * Fix: Add migration to replace JSON null values with empty objects in log entries' data (Sage Abdullah)
  * Docs: Document how to add non-ModelAdmin views to a `ModelAdminGroup` (Onno Timmerman)
  * Docs: Document how to add StructBlock data to a StreamField (Ramon Wenger)
  * Docs: Update ReadTheDocs settings to v2 to resolve urllib3 issue in linkcheck extension (Thibaud Colas)

docs/releases/5.1.md

@@ -124,6 +124,7 @@ This feature was developed by Aman Pandey as part of the Google Summer of Code p
  * Fix preview panel loading issues (Sage Abdullah)
  * Fix `search_promotions` `0004_copy_queries` migration for long-lived Wagtail instances (Sage Abdullah)
  * Guard against `TypeError` in `0088_fix_log_entry_json_timestamps` migration (Sage Abdullah)
+ * Add migration to replace JSON null values with empty objects in log entries' data (Sage Abdullah)
 
 ### Documentation
 

wagtail/migrations/0089_log_entry_data_json_null_to_object.py

@@ -0,0 +1,53 @@
+# Generated by Django 4.2.3 on 2023-07-31 11:00
+
+from django import VERSION as DJANGO_VERSION
+from django.db import migrations, models
+
+
+def replace_json_null_with_empty_object(apps, schema_editor):
+    ModelLogEntry = apps.get_model("wagtailcore.ModelLogEntry")
+    PageLogEntry = apps.get_model("wagtailcore.PageLogEntry")
+
+    # https://github.com/django/django/pull/16245
+    # The way to express a JSON `null` value was changed in Django 4.2.
+    if DJANGO_VERSION >= (4, 2):
+        null = models.Value(None, models.JSONField())
+    else:
+        null = models.Value("null")
+
+    ModelLogEntry.objects.filter(data=null).update(data={})
+    PageLogEntry.objects.filter(data=null).update(data={})
+
+
+class Migration(migrations.Migration):
+    """
+    Replace JSON `null` values with empty JSON objects in the log entry models'
+    `data` field.
+
+    The 0068_log_entry_empty_object migration only handles the case where the
+    `data` (formerly `data_json`) was an empty string (`""`), which was the case
+    for some of the old logs when the field still used a `TextField`. However,
+    in some cases e.g. the `"wagtail.publish"` logs, the `data` was also set to
+    Python `None` that gets serialised to JSON `null`, which is not handled by
+    that migration.
+
+    Empty `data` in logs created after the 0069_log_entry_jsonfield migration
+    (and its accompanying code changes) is normalised to empty JSON objects.
+    So, this migration is only needed for logs created before that migration
+    (i.e. Wagtail 3.0).
+    """
+
+    dependencies = [
+        ("wagtailcore", "0088_fix_log_entry_json_timestamps"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            replace_json_null_with_empty_object,
+            # No reverse operation because once the `null` values are replaced
+            # with empty objects, there's no way to tell which ones were
+            # originally `null` and which ones were originally empty objects.
+            # There is practically no reason to reverse this migration anyway.
+            migrations.RunPython.noop,
+        ),
+    ]