db [nfc]: Assert downgrades are always to current version

And adjust a test to make that true in tests too.

Author: gnprice

lib/model/database.dart

@@ -174,6 +174,13 @@ class AppDatabase extends _$AppDatabase {
           // drop everything from the database and start over.
           // TODO(log): log schema downgrade as an error
           assert(debugLog('Downgrading schema from v$from to v$to.'));
+
+          // In the actual app, the target schema version is always
+          // the latest version as of the code that's being run.
+          // Migrating to earlier versions is useful only for isolating steps
+          // in migration tests; we can forego that for testing downgrades.
+          assert(to == latestSchemaVersion);
+
           await _dropAndCreateAll(m, schemaVersion: to);
           return;
         }

test/model/database_test.dart

@@ -127,25 +127,26 @@ void main() {
     });
 
     test('downgrading', () async {
-      final schema = await verifier.schemaAt(2);
+      final toVersion = AppDatabase.latestSchemaVersion;
+      final schema = await verifier.schemaAt(toVersion);
 
       // This simulates the scenario during development when running the app
       // with a future schema version that has additional tables and columns.
       final before = AppDatabase(schema.newConnection());
       await before.customStatement('CREATE TABLE test_extra (num int)');
       await before.customStatement('ALTER TABLE accounts ADD extra_column int');
       await check(verifier.migrateAndValidate(
-        before, 2, validateDropped: true)).throws<SchemaMismatch>();
+        before, toVersion, validateDropped: true)).throws<SchemaMismatch>();
       // Override the schema version by modifying the underlying value
       // drift internally keeps track of in the database.
       // TODO(drift): Expose a better interface for testing this.
-      await before.customStatement('PRAGMA user_version = 999;');
+      await before.customStatement('PRAGMA user_version = ${toVersion + 1};');
       await before.close();
 
       // Simulate starting up the app, with an older schema version that
       // does not have the extra tables and columns.
       final after = AppDatabase(schema.newConnection());
-      await verifier.migrateAndValidate(after, 2, validateDropped: true);
+      await verifier.migrateAndValidate(after, toVersion, validateDropped: true);
       await after.close();
     });