Fix column references in subquery (#1894)

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

@@ -22,6 +22,10 @@ class _ExistsExpression<T> extends Expression<bool> {
 
   @override
   void writeInto(GenerationContext context) {
+    final outerHasMultipleTables = context.hasMultipleTables;
+    // Inside this subquery, we want to reference columns with their table
+    // to avoid ambiguities when an outer table is referenced.
+    context.hasMultipleTables = true;
     if (_not) {
       context.buffer.write('NOT ');
     }
@@ -30,6 +34,7 @@ class _ExistsExpression<T> extends Expression<bool> {
     context.buffer.write('(');
     _select.writeInto(context);
     context.buffer.write(')');
+    context.hasMultipleTables = outerHasMultipleTables;
   }
 
   @override

drift/test/database/expressions/exists_test.dart

@@ -13,7 +13,7 @@ void main() {
 
     expect(
       existsExpression,
-      generates('EXISTS (SELECT * FROM users WHERE is_awesome = ?)', [1]),
+      generates('EXISTS (SELECT * FROM users WHERE users.is_awesome = ?)', [1]),
     );
   });
 

drift/test/integration_tests/regress_1894_test.dart

@@ -0,0 +1,33 @@
+import 'package:drift/drift.dart';
+import 'package:test/test.dart';
+
+import '../generated/todos.dart';
+import '../test_utils/test_utils.dart';
+
+void main() {
+  test('exists subqueries properly reference columns', () async {
+    final db = TodoDb.connect(testInMemoryDatabase());
+    addTearDown(db.close);
+
+    final nonEmptyId = await db.categories
+        .insertOne(CategoriesCompanion.insert(description: 'category'));
+    await db.todosTable.insertOne(TodosTableCompanion.insert(
+        content: 'entry', category: Value(nonEmptyId)));
+
+    final emptyId = await db.categories.insertOne(
+        CategoriesCompanion.insert(description: 'this category empty YEET'));
+
+    final emptyCategories = await db.emptyCategories();
+
+    expect(emptyCategories, hasLength(1));
+    expect(emptyCategories.single.id, emptyId);
+  });
+}
+
+extension on TodoDb {
+  Future<List<Category>> emptyCategories() {
+    final hasNoTodo = notExistsQuery(select(todosTable)
+      ..where((row) => row.category.equalsExp(categories.id)));
+    return (select(categories)..where((row) => hasNoTodo)).get();
+  }
+}