diff --git a/src/database/migrations/2026_05_10_000001_create_document_translations_and_migrate.php b/src/database/migrations/2026_05_10_000001_create_document_translations_and_migrate.php
index 5d678ec..e0d1ffc 100644
--- a/src/database/migrations/2026_05_10_000001_create_document_translations_and_migrate.php
+++ b/src/database/migrations/2026_05_10_000001_create_document_translations_and_migrate.php
@@ -37,23 +37,28 @@ public function up(): void
         }
 
         // 3. Migrate existing data
+        // Note: step 4 (ALTER TABLE … DROP INDEX) must remain OUTSIDE this
+        // transaction because MySQL's ALTER TABLE causes an implicit commit.
         $defaultLocale = config('app.locale', 'en');
         $now = now();
-        $rows = DB::table('documents')->get();
-        foreach ($rows as $row) {
-            DB::table('document_translations')->insert([
-                'document_id' => $row->id,
-                'locale' => $defaultLocale,
-                'title' => $row->title ?? '',
-                'content' => $row->content ?? '',
-                'rendered_html' => $row->rendered_html,
-                'created_by' => $row->created_by ?? null,
-                'updated_by' => $row->updated_by ?? null,
-                'created_at' => $row->created_at ?? $now,
-                'updated_at' => $row->updated_at ?? $now,
-            ]);
-            DB::table('documents')->where('id', $row->id)->update(['default_locale' => $defaultLocale]);
-        }
+        DB::transaction(function () use ($defaultLocale, $now) {
+            DB::table('documents')->orderBy('id')->chunkById(500, function ($rows) use ($defaultLocale, $now) {
+                foreach ($rows as $row) {
+                    DB::table('document_translations')->insert([
+                        'document_id' => $row->id,
+                        'locale' => $defaultLocale,
+                        'title' => $row->title ?? '',
+                        'content' => $row->content ?? '',
+                        'rendered_html' => $row->rendered_html,
+                        'created_by' => $row->created_by ?? null,
+                        'updated_by' => $row->updated_by ?? null,
+                        'created_at' => $row->created_at ?? $now,
+                        'updated_at' => $row->updated_at ?? $now,
+                    ]);
+                    DB::table('documents')->where('id', $row->id)->update(['default_locale' => $defaultLocale]);
+                }
+            });
+        });
 
         // 4. Drop the old FULLTEXT index on documents (MySQL only)
         if (DB::connection()->getDriverName() === 'mysql') {
@@ -70,9 +75,13 @@ public function up(): void
 
     public function down(): void
     {
-        // Re-add columns
+        // IRREVERSIBLE for non-default-locale translations: only the row matching
+        // each document's default_locale is restored to the legacy columns; any
+        // other-locale translations are dropped along with document_translations.
+
+        // Re-add columns (with the title index that up() expects to drop)
         Schema::table('documents', function (Blueprint $table) {
-            $table->string('title')->nullable()->after('default_locale');
+            $table->string('title')->nullable()->index()->after('default_locale');
             $table->text('content')->nullable()->after('title');
             $table->text('rendered_html')->nullable()->after('content');
         });
diff --git a/src/tests/Feature/DocumentMigrationTest.php b/src/tests/Feature/DocumentMigrationTest.php
index ade25e0..17c1957 100644
--- a/src/tests/Feature/DocumentMigrationTest.php
+++ b/src/tests/Feature/DocumentMigrationTest.php
@@ -2,7 +2,6 @@
 
 namespace Tests\Feature;
 
-use Illuminate\Database\Migrations\Migrator;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Illuminate\Support\Facades\DB;
 use Illuminate\Support\Facades\Schema;
@@ -72,4 +71,44 @@ public function test_document_translations_unique_document_locale(): void
             'updated_at' => now(),
         ]);
     }
+
+    public function test_existing_documents_data_is_copied_to_translations(): void
+    {
+        // Roll the new migration back so the legacy columns exist again
+        \Illuminate\Support\Facades\Artisan::call('migrate:rollback', ['--step' => 1]);
+        $this->assertTrue(\Illuminate\Support\Facades\Schema::hasColumn('documents', 'title'));
+
+        // Seed a legacy document row directly
+        \Illuminate\Support\Facades\DB::table('documents')->insert([
+            'path' => 'Legacy.md',
+            'title' => 'Legacy Title',
+            'slug' => 'legacy-title',
+            'content' => '# Legacy body',
+            'rendered_html' => '<h1>Legacy body</h1>',
+            'file_size' => 0,
+            'file_hash' => str_repeat('0', 64),
+            'file_modified_at' => now(),
+            'created_at' => now(),
+            'updated_at' => now(),
+        ]);
+        $docId = \Illuminate\Support\Facades\DB::table('documents')->where('slug', 'legacy-title')->value('id');
+
+        // Re-run the migration
+        \Illuminate\Support\Facades\Artisan::call('migrate');
+
+        // Verify the data was copied to document_translations
+        $translation = \Illuminate\Support\Facades\DB::table('document_translations')
+            ->where('document_id', $docId)
+            ->first();
+
+        $this->assertNotNull($translation, 'Translation row should have been created from legacy data');
+        $this->assertSame('Legacy Title', $translation->title);
+        $this->assertSame('# Legacy body', $translation->content);
+        $this->assertSame('<h1>Legacy body</h1>', $translation->rendered_html);
+        $this->assertSame(config('app.locale', 'en'), $translation->locale);
+
+        // Verify documents.default_locale was set
+        $defaultLocale = \Illuminate\Support\Facades\DB::table('documents')->where('id', $docId)->value('default_locale');
+        $this->assertSame(config('app.locale', 'en'), $defaultLocale);
+    }
 }