无法将$ match运算符用于具有ObjectId的mongodb/mongoose聚合

相对简单的场景:

我有这个Voucher对象，它具有user属性(类型为ObjectId).我想获取单个用户的所有凭证值的总和.这是我当前的策略，该策略返回一个空数组:

I have this Voucher object which has a user property (of type ObjectId). I want to get the sum of all voucher values for a single user. Here is my current strategy, which returns an empty array:

Voucher.aggregate [
    { $match : { user : new ObjectId(user_id), expires : { $gt : new Date() } } }
    { $group : { _id : null, sum : { $sum : '$value' } } }
], (err, result)->

    console.log err
    console.log result

删除user id的匹配项，并保留expires字段将返回结果.所以问题就变成了user上的匹配是什么问题?

Removing the match for the user id, and leaving the expires field will return results. So the question becomes what is wrong with the match on user?