MongoDartError（MongoDart错误：未知顶级运算符：$filter）：Dart

试图过滤掉任何对象吗？在朋友阵列中？与字段“friends.username”的值不匹配one@one.com

      final userDoc = await store.find({
    '_id': ObjectId.fromHexString(authDetails.subject),
    'friends.username': 'one@one.com',
    '\$filter': {'friends.username': 'one@one.com'}
  }).forEach((element) {
    print(element);
  });

我知道这是可能的，因为我可以使用

      Object newReceived = {
    'username': myUsername,
    'timestamp': addedTime,
  };

  //add your name to their friend request received list
  await store.updateOne({
    '_id': theirId
  }, {
    '\$push': {'friendRequestReceived': newReceived}
  });

mongodb数据库结构如下：

{
"_id" : ObjectId("60b2faa2de1f7691268bf907"),
"email" : "nine5@one.com",
"emailverified" : false,
"username" : "nine5@one.com",
"displayName" : "complete n00b",
"dob" : "",
"points" : 0,
"attended" : 0,
"hosted" : 0,

"friends" : [
    {
        "username" : "one@one.com",
        "timestamp" : "2021-05-29 19:38:58.758325"
    },
    {
        "username" : "nine4@one.com",
        "timestamp" : "2021-05-29 19:39:54.915636"
    }
]

}

我只想获取任何具有“用户名”的文档one@one.com在与“_id”关联的“friends”数组下：ObjectId（“60b2faa2de1f7691268bf907”）。过滤掉所有不需要的用户名，我希望最终使用这个结构只返回来自一个名为messages的数组中的某个用户的消息，该数组名为“_id”：ObjectId（“60b2faa2de1f7691268bf907”）。谢谢