Document how to existing databases on web

docs/lib/snippets/platforms/web.dart

@@ -1,6 +1,7 @@
 import 'package:drift/drift.dart';
 import 'package:drift/wasm.dart';
 import 'package:drift/web.dart';
+import 'package:drift_docs/src/external_apis.dart';
 import 'package:sqlite3/wasm.dart';
 
 typedef _$MyWebDatabase = GeneratedDatabase;
@@ -44,6 +45,25 @@ class MyWebDatabase extends _$MyWebDatabase {
 }
 // #enddocregion connect
 
+DatabaseConnection connectWithExisting() {
+  // #docregion existing
+  return DatabaseConnection.delayed(Future(() async {
+    final result = await WasmDatabase.open(
+      databaseName: 'my_app_db', // prefer to only use valid identifiers here
+      sqlite3Uri: Uri.parse('sqlite3.wasm'),
+      driftWorkerUri: Uri.parse('drift_worker.dart.js'),
+      initializeDatabase: () async {
+        final data = await rootBundle.load('my_database');
+        return data.buffer.asUint8List();
+      },
+    );
+
+    // ...
+    return result.resolvedExecutor;
+  }));
+  // #enddocregion existing
+}
+
 // #docregion migrate-wasm
 // If you've previously opened your database like this
 Future<WasmDatabase> customDatabase() async {

docs/lib/src/external_apis.dart

@@ -0,0 +1,10 @@
+// Stubs for external APIs we want to reference in Snippets without being able
+// to depend on them.
+
+import 'dart:typed_data';
+
+abstract class AssetBundle {
+  Future<ByteData> load(String key);
+}
+
+AssetBundle get rootBundle => throw 'stub';

docs/pages/docs/Platforms/web.md

@@ -217,6 +217,18 @@ export 'unsupported.dart'
 
 A ready example of this construct can also be found [here](https://github.com/simolus3/drift/blob/develop/examples/app/lib/database/connection/connection.dart).
 
+## Using existing databases
+
+You can use existing database with Drift on the web by importing the `initializeDatabase`
+function on `WasmDatabase.open`.
+This function will be called when attempting to open a database that doesn't exist yet,
+allowing the initial sqlite3 file to be provided instead:
+
+{% include "blocks/snippet" snippets = snippets name = "existing" %}
+
+Please be aware that this only works for the default journal mode, WAL is not supported
+on the web and WAL databases can't be imported with `initializeDatabase` either.
+
 ## Examples
 
 The drift repository contains a two small web applications using drift on the web: