Angularfire无法检索密钥

I have this code block here;

我这里有这个代码块;

 return this.db.list(`users/${currentId}/chatThreads/`)
      .map((thread) => {
        thread.map((t) => {
          let x = {
            key: t, // <- Undefined? Where's the key
            count: t.$value,
          }
          console.log(x);
          return x;
        })
      });
 re