Support schema downgrades (#2336)

drift/CHANGELOG.md

@@ -5,6 +5,8 @@
   statements.
 - Add `rowid` parameter to companions for tables with rowids that don't have a
   visible alias for the rowid.
+- After opening a database with a higher schema version than the current one set
+  in the database class, the schema version in the database will now be downgraded.
 
 ## 2.5.0
 

drift/lib/src/runtime/executor/helpers/engines.dart

@@ -458,7 +458,7 @@ class DelegatedDatabase extends _BaseExecutor {
     await user.beforeOpen(_BeforeOpeningExecutor(this), openingDetails);
 
     if (versionDelegate is DynamicVersionDelegate &&
-        (oldVersion == null || oldVersion < currentVersion)) {
+        oldVersion != currentVersion) {
       // set version now, after migrations ran successfully
       await versionDelegate.setSchemaVersion(currentVersion);
     }

drift/test/engines/delegated_database_test.dart

@@ -103,7 +103,7 @@ void main() {
       verify(delegate.open(userDb));
       verifyNever(delegate.runCustom(any, any));
       verify(version.schemaVersion);
-      // Running migrations from version 3 to 3
+      // Not running migrations from version 3 to 3
       verifyNever(version.setSchemaVersion(3));
 
       when(version.schemaVersion).thenAnswer((_) => Future.value(2));
@@ -127,6 +127,17 @@ void main() {
 
       verify(delegate.runCustom('updated', argThat(equals([1, 3]))));
     });
+
+    test('handles database downgrades', () async {
+      final version = MockDynamicVersionDelegate();
+      when(version.schemaVersion).thenAnswer((_) => Future.value(4));
+      when(delegate.versionDelegate).thenReturn(version);
+      await db.ensureOpen(userDb);
+
+      verify(delegate.open(userDb));
+      verify(delegate.runCustom('updated', argThat(equals([4, 3]))));
+      verify(version.setSchemaVersion(3));
+    });
   });
 
   group('transactions', () {