@@ -0,0 +1,18 @@
+---
+'@objectstack/driver-sql': patch
+---
+
+fix(driver-sql): schema sync never drops an existing table that holds rows; a `_id`-keyed table with rows is refused, naming the table and the column
+
+Clause-②: yes (narrowing) — `SqlDriver.initObjects` / `syncSchema` refuse one more case that used to succeed: an existing table that has a `_id` column, no `id` column and at least one row. It used to be dropped and recreated empty; it is now refused with an `ExistingTableAdoptionRefusal` naming the table, the column and the row count, and nothing is changed.
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (no-migration-prescription) No key of any schema is added, removed, renamed or re-shaped, and no stored metadata changes: what moves is what schema sync does to a database table it finds, and the refused table is left exactly as it was, so there is nothing for a migration to rewrite. -->
+
+**BREAKING** accept-set narrowing in schema sync (`@objectstack/driver-sql`): a table the sync would have rebuilt is refused while it holds rows.
+
+- **Before.** Schema sync adopts an existing table by adding the columns the object declares and the table lacks, rows kept. A table keyed by `_id` with no `id` column (the storage key of the ObjectQL driver this one was ported from) was instead dropped and recreated as the object declares, whatever it held. Storing an object on a managed datasource over an existing database reached it: an object named after such a table deleted its rows, and the save answered success.
+- **Now.** That rebuild runs only on an empty table, as before. A table that holds rows is refused before anything is written: no column is added, no index is built, no row is touched. The refusal is thrown from `initObjects` and reaches the caller of the sync. The metadata save, the boot sync and a datasource connect report a refused sync in the server log at `error`, followed by this message, as they do for any refused sync.
+- **Who moves.** Only a deployment storing an object on a managed SQL datasource whose table of the same name is keyed by `_id` and holds rows. No platform table has that shape.
+
+**The remedy.** To keep the rows and use the table, rename its `_id` column to `id`, then save the object again (or restart). To read the table as it is, bind the object through a datasource whose `schemaMode` is `external`. If the rows are not needed, empty the table and save again; the sync then rebuilds it.
@@ -0,0 +1,59 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * Schema sync never drops a table that holds rows.
+ *
+ * `initObjects` adopts an existing table by adding the columns the object
+ * declares and it lacks — rows kept. One shape it cannot adopt that way: a
+ * table keyed by `_id` with no `id` column, the storage key of the ObjectQL
+ * driver this one was ported from. Every row this driver writes and reads is
+ * keyed by `id`, so for that shape the sync rebuilds the table: drop, then
+ * create it as the object declares. On an empty table that loses nothing and
+ * still happens.
+ *
+ * On a table that holds rows the rebuild deleted them, with no warning. The
+ * table did not have to be the platform's: a managed datasource over an
+ * existing database is reached by storing an object whose name matches one of
+ * its tables, and the save went on to answer success. So the rebuild is now
+ * gated on the row count, and a table holding rows is refused instead — before
+ * anything is written: no column added, no index built, no row touched. The
+ * refusal names the table and the column, and says what the operator can do.
+ *
+ * It carries no ADR-0112 `code`. No door answers this refusal on the wire: the
+ * metadata save door reports a refused schema sync in the server log at
+ * `error` and answers the save (`ensureObjectStorage`), the boot sync and a
+ * datasource connect do the same, and `os migrate apply` prints the message.
+ * A code a driver stamps carries a row in the error-code ledger; this refusal
+ * has no door to serve one through.
+ */
+
+/** Thrown by `SqlDriver.initObjects` (`sql-driver-rotation.ts`) — its one throw site. */
+export class ExistingTableAdoptionRefusal extends Error {
+  constructor(
+    /** The existing table the sync was refused for. */
+    readonly table: string,
+    /** The column that makes its shape one the sync can only rebuild. */
+    readonly column: string,
+    /** How many rows it holds — never zero: an empty table is rebuilt. */
+    readonly rows: number,
+  ) {
+    super(renderExistingTableAdoptionRefusal(table, column, rows));
+    this.name = 'ExistingTableAdoptionRefusal';
+  }
+}
+
+/**
+ * The refusal's prose: what was refused and why, what was left alone, and the
+ * three ways forward. Exported so the pins read the renderer the driver throws
+ * through, never a copy of its text.
+ */
+export function renderExistingTableAdoptionRefusal(table: string, column: string, rows: number): string {
+  return (
+    `Schema sync REFUSED for the existing table '${table}': it has a '${column}' column and no 'id' column, ` +
+    `and it holds ${rows} row${rows === 1 ? '' : 's'}. ObjectStack keys every row by 'id', so it could adopt ` +
+    `this table only by dropping and recreating it, which would delete those rows. Nothing was changed: no ` +
+    `column was added and no row was touched. To keep the rows and use the table, rename its '${column}' ` +
+    `column to 'id' and save the object again; to read the table as it is, bind the object through a ` +
+    `datasource whose schemaMode is 'external'; if the rows are not needed, empty the table and save again.`
+  );
+}
@@ -36,6 +36,7 @@ import { FILE_REFERENCE_TYPES, NUMERIC_VALUE_TYPES, isTenancyDisabled, parseAuto
 import type { AutonumberToken } from '@objectstack/spec/data';
 import { aggregandColumnClass } from '@objectstack/core';
 import { StorageNameMapping, SystemObjectName } from '@objectstack/spec/system';
+import { ExistingTableAdoptionRefusal } from './existing-table-refusal.js';
 import { indexedKeyColumns, isUniqueScopeDeclared } from './schema-drift.js';
 import {
   AUDIT_TIMESTAMP_COLUMNS, MYSQL_DATETIME_PRECISION, NUMERIC_SCALAR_TYPES, declaredValueShapes,
@@ -458,7 +459,7 @@ export class RotationFacet {
       let exists = await this.knex.schema.hasTable(tableName);
       // Recorded BEFORE the legacy-`_id` rebuild below can flip `exists`: a
       // table that was here when we arrived stays "found" even if we then drop
-      // and recreate it. Re-syncing a table we made ourselves this boot — boot
+      // and recreate it (or refuse it). Re-syncing a table we made ourselves this boot — boot
       // runs the sync more than once, and objects registered later sync on
       // demand — is not evidence of anything preceding us, so it is ignored.
       // See `getSchemaSyncStats`.
@@ -469,7 +470,17 @@ export class RotationFacet {
         const columnInfo = await this.knex(tableName).columnInfo();
         const existingColumns = Object.keys(columnInfo);
 
+        // A table keyed by `_id` with no `id` (the ObjectQL driver's storage
+        // key) is the one shape the additive path below cannot adopt, so it is
+        // rebuilt — but only while it is EMPTY. [#30562] Schema sync never
+        // drops a table that holds rows: on a managed datasource over an
+        // existing database that table is the customer's, and the drop deleted
+        // its rows behind a save that answered success. Refused here, before
+        // any DDL, so nothing about the table changes. See
+        // `existing-table-refusal.ts`.
         if (existingColumns.includes('_id') && !existingColumns.includes('id')) {
+          const [{ rows }] = await this.knex(tableName).count({ rows: '*' });
+          if (Number(rows) > 0) throw new ExistingTableAdoptionRefusal(tableName, '_id', Number(rows));
           await this.knex.schema.dropTable(tableName);
           exists = false;
         }
 
@@ -2,6 +2,8 @@
 
 import { describe, it, expect, beforeEach, afterEach } from 'vitest';
 import { SqlDriver } from '../src/index.js';
+import { ExistingTableAdoptionRefusal } from './existing-table-refusal.js';
+import { DIALECT_CELLS, declareDialectCell, type DialectCell } from './live-dialect-matrix.testkit.js';
 
 describe('SqlDriver Schema Sync (SQLite)', () => {
   let driver: SqlDriver;
@@ -494,3 +496,123 @@ describe('SqlDriver Schema Sync (SQLite)', () => {
     expect(existsFqn).toBe(false);
   });
 });
+
+/**
+ * Schema sync never drops a table that holds rows.
+ *
+ * `initObjects` rebuilds an existing table keyed by `_id` with no `id` — the
+ * one shape it cannot adopt by adding columns. Measured through
+ * `PUT /api/v1/meta/object/NAME` on a managed SQLite datasource over an
+ * existing file: such a table holding 2 rows was dropped and recreated empty,
+ * and the save answered success. The rebuild now runs only on an empty table;
+ * a table holding rows is refused before anything is written.
+ *
+ * Driven through `syncSchema`, the call the engine makes per object. Across
+ * every dialect cell: SQLite always, Postgres and MySQL where the runner
+ * provisions them (the `Temporal Conformance (live PG + MySQL)` job runs this
+ * package against both).
+ */
+const ROW_GUARD_MATRIX = 'schema sync never drops a table holding rows';
+
+/** A table this file owns; the schema it lands in is per-file on a live cell. */
+const LEGACY_TABLE = 'os30562_legacy_items';
+
+const LEGACY_OBJECT = {
+  name: LEGACY_TABLE,
+  fields: { title: { type: 'string' }, qty: { type: 'integer' } },
+};
+
+const LEGACY_ROWS = [
+  { _id: 'a1', title: 'Bolt' },
+  { _id: 'a2', title: 'Nut' },
+];
+
+function declareRowGuardPins(cell: DialectCell): void {
+  describe(`${ROW_GUARD_MATRIX} — ${cell.label}`, () => {
+    let driver: SqlDriver;
+    let knex: any;
+
+    /** The table as it stands: its column names, and every row in key order. */
+    const snapshot = async (key: string) => ({
+      columns: Object.keys(await knex(LEGACY_TABLE).columnInfo()).sort(),
+      rows: await knex(LEGACY_TABLE).select('*').orderBy(key),
+    });
+
+    beforeEach(async () => {
+      driver = new SqlDriver(cell.config());
+      knex = (driver as any).knex;
+      await knex.schema.dropTableIfExists(LEGACY_TABLE);
+    });
+
+    afterEach(async () => {
+      await knex.schema.dropTableIfExists(LEGACY_TABLE).catch(() => {});
+      await driver.disconnect();
+    });
+
+    it('refuses a `_id`-keyed table holding rows, naming the table and the column, and changes nothing', async () => {
+      await knex.schema.createTable(LEGACY_TABLE, (t: any) => {
+        t.string('_id').primary();
+        t.string('title');
+      });
+      await knex(LEGACY_TABLE).insert(LEGACY_ROWS);
+      const before = await snapshot('_id');
+      // Non-vacuous: the shape is the one the sync rebuilds, and it holds rows.
+      expect(before.columns).toEqual(['_id', 'title']);
+      expect(before.rows).toHaveLength(2);
+
+      const refusal = await driver.syncSchema(LEGACY_TABLE, LEGACY_OBJECT).then(() => null, (e: unknown) => e);
+
+      expect(refusal).toBeInstanceOf(ExistingTableAdoptionRefusal);
+      expect(refusal).toMatchObject({
+        name: 'ExistingTableAdoptionRefusal',
+        table: LEGACY_TABLE,
+        column: '_id',
+        rows: 2,
+      });
+      const { message } = refusal as Error;
+      expect(message).toContain(`'${LEGACY_TABLE}'`);
+      expect(message).toContain(`'_id'`);
+
+      // Nothing was changed: the table is still there, with the same columns
+      // (no declared column added) and the same rows.
+      expect(await knex.schema.hasTable(LEGACY_TABLE)).toBe(true);
+      expect(await snapshot('_id')).toEqual(before);
+    });
+
+    it('CONTROL: rebuilds an empty `_id`-keyed table as the object declares, as before', async () => {
+      await knex.schema.createTable(LEGACY_TABLE, (t: any) => {
+        t.string('_id').primary();
+        t.string('title');
+      });
+
+      await driver.syncSchema(LEGACY_TABLE, LEGACY_OBJECT);
+
+      const after = await snapshot('id');
+      expect(after.columns).toContain('id');
+      expect(after.columns).not.toContain('_id');
+      expect(after.columns).toEqual(expect.arrayContaining(['created_at', 'updated_at', 'title', 'qty']));
+      expect(after.rows).toEqual([]);
+    });
+
+    it('CONTROL: adopts an `id`-keyed table holding rows by adding the declared columns, rows kept', async () => {
+      await knex.schema.createTable(LEGACY_TABLE, (t: any) => {
+        t.string('id').primary();
+        t.string('title');
+      });
+      await knex(LEGACY_TABLE).insert(LEGACY_ROWS.map(({ _id, title }) => ({ id: _id, title })));
+
+      await driver.syncSchema(LEGACY_TABLE, LEGACY_OBJECT);
+
+      const after = await snapshot('id');
+      expect(after.columns).toEqual(['id', 'qty', 'title']);
+      expect(after.rows.map(({ id, title }: any) => ({ id, title }))).toEqual([
+        { id: 'a1', title: 'Bolt' },
+        { id: 'a2', title: 'Nut' },
+      ]);
+    });
+  });
+}
+
+for (const cell of DIALECT_CELLS) {
+  declareDialectCell(cell, ROW_GUARD_MATRIX, declareRowGuardPins);
+}