Add example on existing databases

docs/content/en/docs/Examples/existing_databases.md

@@ -0,0 +1,62 @@
+---
+title: "Existing databases"
+description: Using moor with an existing database
+---
+
+You can use moor with a pre-propulated database that you ship with your app.
+
+## Including the database
+
+First, create the sqlite3 database you want to ship with your app.
+You can create a database with the [sqlite3 CLI tool](https://sqlite.org/cli.html)
+on your development machine.
+Of course, you can also create the database programmatically by using a library
+like [sqlite3](https://pub.dev/packages/sqlite3) (or even moor itself).
+
+To ship that database to users, you can include it as a [flutter asset](https://flutter.dev/docs/development/ui/assets-and-images).
+Simply include it in your pubspec:
+
+```yaml
+flutter:
+  assets:
+    - assets/my_database.db
+```
+
+## Extracting the database
+
+To initialize the database before using moor, you need to extract the asset from your
+app onto the device.
+In moor, you can use a [LazyDatabase](https://pub.dev/documentation/moor/latest/moor/LazyDatabase-class.html)
+to perform that work just before your moor database is opened:
+
+```dart
+import 'package:moor/moor.dart';
+import 'package:flutter/services.dart' show rootBundle;
+
+LazyDatabase _openConnection() {
+  return LazyDatabase(() async {
+    // put the database file, called db.sqlite here, into the documents folder
+    // for your app.
+    final dbFolder = await getApplicationDocumentsDirectory();
+    final file = File(p.join(dbFolder.path, 'app.db'));
+    
+    if (!await file.exists()) {
+        // Extract the pre-populated datbaase file from assets
+        final blob = await rootBundle.load('assets/my_database.db');
+        await file.writeAsBytes(blob);
+    }
+
+    return VmDatabase(file);
+  });
+}
+```
+
+Finally, use that method to open your database:
+
+```dart
+@UseMoor(tables: [Todos, Categories])
+class MyDatabase extends _$MyDatabase {
+  MyDatabase() : super(_openConnection());
+
+  // ...
+```