Javascript 如何在IndexedDB中按属性查询对象？

我有一个IndexedDB对象存储，它存储以下结构的用户对象：

{ id: 1, name: "Peter", role: "admin", password: "someHash"}

对象存储的创建如下所示：

request.onupgradeneeded = function(event){
    var db = request.result;
    var objectStore = db.createObjectStore("users", { keyPath: "id", autoIncrement: true });

    objectStore.createIndex("uniqueNameIndex", "name", { unique: true  });
    objectStore.createIndex("roleIndex",       "role", { unique: false });
};

我知道如何通过对象的键（id）检索对象，并且我还能够使用光标遍历所有对象

我现在想做的是获取所有具有特定角色的用户-例如检索所有管理员，而不迭代整个对象存储

我该怎么做

function forEachAdmin(db, callback) {
  var ROLE_ADMIN = 'admin';
  var tx = db.transaction('users');
  var store = tx.objectStore('users');
  var index = store.index('roleIndex');
  var range = IDBKeyRange.only(ROLE_ADMIN);
  var request = index.openCursor(range);
  request.onsuccess = function() {
    var cursor = this.result;
    if(cursor) {
      var adminUser = cursor.value;
      callback(adminUser);
      cursor.continue();
    } else {
      // No admins found or all admins iterated
    }
  };
}

function handleAdmin(user) {
  console.log('Doing something with admin user %o', user);
}

var request = indexedDB.open('dbname');
request.onsuccess = function() {
  var db = this.result;
  forEachAdmin(db, handleAdmin);
};