anchor.diff:  
           create unique index $_tableCacheObject${CacheObject.columnKey} 
             on $_tableCacheObject (${CacheObject.columnKey});
  +        ''');
  +      }
  +      if (oldVersion <= 2) {
  +        await db.execute('''
  +        alter table $_tableCacheObject 
  +        add ${CacheObject.columnLength} integer;
           ''');
         }
       });
  
positive.code:       if (oldVersion <= 1) {
          await db.execute('''
          alter table $_tableCacheObject 
          add ${CacheObject.columnKey} text,
              ${CacheObject.columnLength} integer;
  
          update $_tableCacheObject 
            set ${CacheObject.columnKey} = ${CacheObject.columnUrl}
  
negative.code: 
    @override
    Future open() async {
      db = await openDatabase(path, version: 3,
          onCreate: (Database db, int version) async {
        await db.execute('''
        create table $_tableCacheObject ( 
  