From 43bc683d80c620d9752e68a317d51891a00a060a Mon Sep 17 00:00:00 2001
From: Braden MacDonald <braden@opencraft.com>
Date: Wed, 27 Oct 2021 14:51:55 -0700
Subject: [PATCH] feat: Add a data migration to copy all course index data into
 MySQL

---
 .../migrations/0002_data_migration.py         | 61 +++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 common/djangoapps/split_modulestore_django/migrations/0002_data_migration.py

diff --git a/common/djangoapps/split_modulestore_django/migrations/0002_data_migration.py b/common/djangoapps/split_modulestore_django/migrations/0002_data_migration.py
new file mode 100644
index 0000000000..3d33e53ab5
--- /dev/null
+++ b/common/djangoapps/split_modulestore_django/migrations/0002_data_migration.py
@@ -0,0 +1,61 @@
+from django.db import migrations, models
+from django.db.utils import IntegrityError
+
+
+from xmodule.modulestore import ModuleStoreEnum
+from xmodule.modulestore.django import modulestore
+
+from ..models import SplitModulestoreCourseIndex as SplitModulestoreCourseIndex_Real
+
+
+def forwards_func(apps, schema_editor):
+    """
+    Copy all course index data from MongoDB to MySQL, unless it's already present in MySQL.
+
+    This migration is used as part of an upgrade path from storing course indexes in MongoDB to storing them in MySQL.
+    On edX.org, we began writing to MySQL+MongoDB before we deployed this migration, so some courses are already in
+    MySQL. But any courses that haven't been modified recently would only be in MongoDB and need to be copied over to
+    MySQL before we can switch reading course indexes to MySQL.
+    """
+    db_alias = schema_editor.connection.alias
+    SplitModulestoreCourseIndex = apps.get_model("split_modulestore_django", "SplitModulestoreCourseIndex")
+    split_modulestore = modulestore()._get_modulestore_by_type(ModuleStoreEnum.Type.split)
+
+    for course_index in split_modulestore.db_connection.find_matching_course_indexes(force_mongo=True):
+        data = SplitModulestoreCourseIndex_Real.fields_from_v1_schema(course_index)
+        course_id = data["course_id"]
+
+        try:
+            mysql_entry = SplitModulestoreCourseIndex.objects.get(course_id=course_id)
+            # This course index ("active version") already exists in MySQL.
+            # Let's just make sure it's the latest version. If the MongoDB somehow contains a newer version, something
+            # has gone wrong and we should investigate to ensure we're not losing any data.
+            if mysql_entry.edited_on < data["edited_on"]:
+                raise ValueError(
+                    f"Course {course_id} already exists in MySQL but the MongoDB version is newer. "
+                    "That's unexpected because since the course index table was added to MySQL, there has never been a "
+                    "time when we would write course_indexes updates only to MongoDB without also writing to MySQL."
+                )
+        except SplitModulestoreCourseIndex.DoesNotExist:
+            # This course exists in MongoDB but hasn't yet been migrated to MySQL. Do that now.
+            SplitModulestoreCourseIndex(**data).save(using=db_alias)
+
+def reverse_func(apps, schema_editor):
+    """
+    Reversing the data migration is a no-op, because edX.org used a migration path path that started with writing to
+    both MySQL+MongoDB while still reading from MongoDB, then later executed this data migration, then later cut over to
+    reading from MySQL only. If we reversed this by deleting all entries, it would undo any writes that took place
+    before this data migration, which are unrelated.
+    """
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('split_modulestore_django', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunPython(forwards_func, reverse_func),
+    ]