如何在包含文档数组的字段中找到没有其他用户 ID 的用户？

Question

我正在构建一个 facebook 克隆来了解后端，我需要我的前端来显示尚未发送朋友请求的客户端的用户。

User.findOne({ _id: req.params.id })
        .then((user) => {
            User.find(
                {
                    _id: { $nin: user.friends, $ne: user._id },
                    friend_requests: { from: { $ne: user._id }, to: { $ne: user._id } } // <-- I thought this would work but it gives me a cast error
                },
                'first_name last_name profile_picture',
            )
                .limit(5)
                .populate({ path: 'friend_requests', populate: { path: 'from', model: 'User' } })
                .then((people) => {
                    // people here needs to exclude whatever users the client has already sent friend requests to 
                });

我的朋友请求架构是这样的

const FriendRequestSchema = new Schema({
    status: { type: String, enum: ['Accepted', 'Pending', 'Declined'] },
    from: { type: Schema.Types.ObjectId, ref: 'User' },
    to: { type: Schema.Types.ObjectId, ref: 'User' },
});

它们保存在用户 model 中，如下所示：

friend_requests: [{ type: Schema.Types.ObjectId, ref: 'FriendRequest' }]