Make groupConcat nullable in Dart (closes #1628)

drift/CHANGELOG.md

@@ -1,7 +1,11 @@
-## 1.3.0
+## (unreleased)
 
 - Add the `from(table)` method to generated databases. It can be used to write
   common queries more concisely.
+- Make `groupConcat` nullable in the Dart API.
+- Throw an exception in a `NativeDatabase` when multiple statements are run in
+  a single call. In previous versions, parts of the SQL string would otherwise
+  be ignored.
 
 ## 1.2.0
 

drift/lib/src/runtime/query_builder/expressions/aggregate.dart

@@ -33,12 +33,13 @@ extension BaseAggregate<DT> on Expression<DT> {
   /// Returns the concatenation of all non-null values in the current group,
   /// joined by the [separator].
   ///
-  /// The order of the concatenated elements is arbitrary.
+  /// The order of the concatenated elements is arbitrary. If no non-null values
+  /// exist in the group, `NULL` is returned.
   ///
   /// See also:
   ///  - the sqlite documentation: https://www.sqlite.org/lang_aggfunc.html#groupconcat
   ///  - the conceptually similar [Iterable.join]
-  Expression<String> groupConcat({String separator = ','}) {
+  Expression<String?> groupConcat({String separator = ','}) {
     const sqliteDefaultSeparator = ',';
     if (separator == sqliteDefaultSeparator) {
       return _AggregateExpression('GROUP_CONCAT', this);

drift/test/expressions/expressions_integration_test.dart

@@ -1,5 +1,5 @@
 @TestOn('vm')
-import 'package:drift/drift.dart';
+import 'package:drift/drift.dart' hide isNull;
 import 'package:drift/native.dart';
 import 'package:test/test.dart';
 
@@ -139,6 +139,16 @@ void main() {
     expect(eval(noMatch), completion(isFalse));
   });
 
+  test('groupConcat is nullable', () async {
+    final ids = db.users.id.groupConcat();
+    final query = db.selectOnly(db.users)
+      ..where(db.users.id.equals(999))
+      ..addColumns([ids]);
+
+    final result = await query.getSingle();
+    expect(result.read(ids), isNull);
+  });
+
   test('subqueries cause updates to stream queries', () async {
     await db
         .into(db.categories)