@@ -0,0 +1,28 @@
+---
+'@objectstack/metadata-protocol': patch
+'@objectstack/metadata-core': patch
+---
+
+fix(metadata-protocol): two locales of one email template saved through the metadata door both survive a restart — an overlay row is keyed by its bundle member (#22877)
+
+Clause-②: no
+
+**What an administrator sees.** Saving the same email template in two languages through the metadata door (`PUT /api/v1/meta/email_template/:name`, the Studio editor's save) keeps both. Before, the store kept one overlay row per template name, so the second language's save replaced the first language's row: both read back until the next restart, and then the first language mailed the package wording again. Each language now keeps its own wording across restarts, and each send in that language uses it.
+
+**What changed in the store.**
+
+- `sys_metadata` gains one nullable column, `bundle_member`. It holds the member of a metadata type whose identity is a pair — today `email_template`, identified by `(name, locale)`. NULL is the canonical member (`en-US`, the locale a send falls back to). Every other row stays NULL, so rows of every other type, and an `en-US` template row, are byte-identical to before.
+- The value is taken from the saved body's `locale`; it is never authored. A body that declares no `locale` is the `en-US` member.
+- The overlay uniqueness index becomes `(type, name, organization_id, COALESCE(package_id, ''), COALESCE(bundle_member, ''))`, for active rows and for draft rows, and the declared index in `metadata-core` carries the column too. A second row for the same template, language, package and state is refused by the index.
+
+**What a read or a door addresses.** A save addresses the language its body names. A read, delete, publish, history or revert by name that names no language addresses the `en-US` member: `GET /meta/email_template/:name` serves the `en-US` wording (the stored one when it was customized, else the package's), where before it served whichever row the name had. The layered list and the send path serve every language as before. A non-`en-US` language's own customization is reached through the list, and is replaced by a save that names its `locale`; a by-name reset (`DELETE`) removes the `en-US` customization only. The lineage counter stays one per template name, so a version number names one saved snapshot, and a revert by name restores only a version of the `en-US` member. Resetting, publishing or reading a non-`en-US` language by name needs the by-name member selector, which arrives with stage 2 of #22877; until then those doors address the `en-US` member.
+
+**What happens on the first boot after upgrading.** Nothing to configure.
+
+1. The schema sync adds the `bundle_member` column.
+2. Boot hydration gives every `email_template` row stored before the column existed the member its body declares, so a row an earlier save left holding a second language's wording becomes that language's row, not the `en-US` one. Rows of every other type are not read or written.
+3. The first metadata write rebuilds the overlay index with the new key, through the existing probe-then-replace order. Adding a key part only relaxes uniqueness, so no rows a store already holds can block the rebuild; a store that was already blocked is reported as before, with the duplicate-listing query and `os migrate duplicates`.
+
+**On a database without partial indexes (MySQL).** The rebuild above is refused there, as before, and reported in the server log at the first metadata write. A store that booted an earlier version keeps the four-column declared index under `idx_sys_metadata_overlay_active`, because the schema sync skips an index whose name already exists. That index has never constrained an environment-wide overlay row: those rows store no organization, and a UNIQUE key does not collide NULLs. So two languages of one template coexist there too, and no save is refused. `os migrate plan` lists the declaration change on that index as a destructive `recreate_index`. To bring the index to the five-column declaration, run `os migrate apply --allow-destructive`. A fresh store gets the five-column declaration.
+
+`os migrate duplicates` boots read-only and adds no column. Run against a store this version has not booted yet, it lists the two overlay indexes with every row read as the empty member — the value the column will hold — so a duplicate that will block the rebuild is still reported, and the entry's `duplicateSql` is the member-less statement that ran there.
@@ -93,6 +93,27 @@ export const SysMetadataObject = ObjectSchema.create({
       defaultValue: 'platform',
     }),
 
+    /**
+     * The bundle member this row overlays, for a metadata type whose identity
+     * is a pair (`ITEM_KEY_DISCRIMINATORS` in `../item-key-discriminators.ts`,
+     * today `email_template`'s `(name, locale)`).
+     *
+     * NULL is the type's CANONICAL member, which is also every row of a type
+     * whose identity is `name` alone — so those rows never carry a value and
+     * stay byte-identical. A non-canonical member stores the value the body
+     * declares (`itemDiscriminator`), so two locales of one template are two
+     * rows. Written by the overlay repository from the body, never authored;
+     * part of the overlay uniqueness key below and of the runtime index
+     * (`metadata-protocol`'s `migrations/overlay-index.ts`).
+     */
+    bundle_member: Field.text({
+      label: 'Bundle Member',
+      required: false,
+      maxLength: 100,
+      description:
+        'Bundle member (e.g. the locale of an email template) this overlay row customizes. Null = the canonical member.',
+    }),
+
     /** JSON payload — the actual metadata configuration */
     metadata: Field.textarea({
       label: 'Metadata',
@@ -200,11 +221,13 @@ export const SysMetadataObject = ObjectSchema.create({
 
   indexes: [
     // ADR-0005 (revised 2026-05) + ADR-0048: overlay uniqueness is scoped by
-    // (type, name, organization_id, package_id). `package_id` is part of the
-    // discriminator so two installed packages shipping the same `type`/`name`
-    // each get their OWN customization row (a package-less / global overlay
-    // uses NULL). environment_id is deprecated and not part of the
-    // discriminator.
+    // (type, name, organization_id, package_id, bundle_member). `package_id` is
+    // part of the discriminator so two installed packages shipping the same
+    // `type`/`name` each get their OWN customization row (a package-less /
+    // global overlay uses NULL). `bundle_member` is the bundled type's member
+    // (#22877): two locales of one email template are two overlay rows; NULL
+    // is the canonical member and every undiscriminated type's row.
+    // environment_id is deprecated and not part of the discriminator.
     //
     // ⚠️ The active-row restriction is NOT declared here, and never was in any
     // effective sense: this entry carried `partial: "state = 'active'"` until
@@ -217,15 +240,16 @@ export const SysMetadataObject = ObjectSchema.create({
     // What actually delivers active-row scoping is the runtime layer:
     // `protocol.ts ensureOverlayIndex` issues a DROP-then-CREATE migration for
     // `CREATE UNIQUE INDEX … ON sys_metadata (type, name, organization_id,
-    // COALESCE(package_id,'')) WHERE state = 'active'` — which additionally
-    // fixes NULL-distinctness for package-less rows, something this
-    // declaration could not express either. This entry is therefore the
+    // COALESCE(package_id,''), COALESCE(bundle_member,'')) WHERE state =
+    // 'active'` — which additionally fixes NULL-distinctness for package-less
+    // rows and canonical members, something this declaration could not
+    // express either. This entry is therefore the
     // coarser fallback that a driver without that runtime migration gets: it
     // still prevents duplicate ACTIVE overlays, at the cost of also colliding
     // with archived/reset rows.
     {
       name: 'idx_sys_metadata_overlay_active',
-      fields: ['type', 'name', 'organization_id', 'package_id'],
+      fields: ['type', 'name', 'organization_id', 'package_id', 'bundle_member'],
       unique: 'global',
     },
     { name: 'idx_sys_metadata_org_type', fields: ['organization_id', 'type'] },