fix(db): use partialFilterExpression for stripeCustomerId unique index

packages/kal-db/migrations/20260407000001_add_stripe_fields.js

@@ -19,9 +19,14 @@ export const up = async (db, client) => {
   );
 
   // Create index on stripeCustomerId for webhook lookups
-  await db
-    .collection("users")
-    .createIndex({ stripeCustomerId: 1 }, { sparse: true, unique: true });
+  // Use partialFilterExpression so null values are excluded from uniqueness
+  await db.collection("users").createIndex(
+    { stripeCustomerId: 1 },
+    {
+      unique: true,
+      partialFilterExpression: { stripeCustomerId: { $type: "string" } },
+    }
+  );
 };
 
 export const down = async (db, client) => {

packages/kal-db/migrations/20260407000004_fix_stripe_customer_id_index.js

@@ -0,0 +1,48 @@
+/**
+ * Migration: Fix stripeCustomerId index using partialFilterExpression
+ *
+ * Safety net for environments where migration 000001 partially applied —
+ * the updateMany set stripeCustomerId: null on all users, but the
+ * createIndex with { sparse: true, unique: true } failed because sparse
+ * indexes still include documents where the field is explicitly null.
+ *
+ * This drops any existing stripeCustomerId index and recreates it with
+ * a partialFilterExpression that only indexes documents where
+ * stripeCustomerId is a string — completely excluding null/missing values
+ * from the uniqueness constraint.
+ */
+
+export const up = async (db, client) => {
+  // Drop the existing index if it exists (may be sparse or broken)
+  await db
+    .collection("users")
+    .dropIndex("stripeCustomerId_1")
+    .catch(() => {});
+
+  // Recreate with partialFilterExpression — only enforce uniqueness on real Stripe IDs
+  await db.collection("users").createIndex(
+    { stripeCustomerId: 1 },
+    {
+      unique: true,
+      partialFilterExpression: { stripeCustomerId: { $type: "string" } },
+    }
+  );
+
+  console.log(
+    "✅ Recreated stripeCustomerId index with partialFilterExpression (string only)"
+  );
+};
+
+export const down = async (db, client) => {
+  // Revert to the sparse unique index from the original migration
+  await db
+    .collection("users")
+    .dropIndex("stripeCustomerId_1")
+    .catch(() => {});
+
+  await db
+    .collection("users")
+    .createIndex({ stripeCustomerId: 1 }, { sparse: true, unique: true });
+
+  console.log("✅ Reverted stripeCustomerId index to sparse unique");
+};