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