您基本上使用 $cond 在这里:

You basically use $setIsSubset and $cond here:

var userId= "5add82620d7f5b38240c63d4";

Model.aggregate([
  { "$group": {
    "_id": null,
    "numberOfUnreadOffers": {
      "$sum": {
        "$cond": {
          "if": { 
            "$setIsSubset": [[mongoose.Types.ObjectId(userId)], "$readBy"]
          },
          "then": 0,
          "else": 1
        }
      }
    }
  }}
]) 

当然，您还需要使用mongoose.Types.ObjectId从字符串"投射"到有效的ObjectId值.

Of course you also need to "cast" using mongoose.Types.ObjectId from the "string" to a valid ObjectId value.

但是，实际上，您可以通过简单的 count() 来获得更好的性能:

But really you get better performance from a simple count() instead:

Model.count({ "readBy": { "$ne": userId } })

因此，您实际上根本不应该使用.aggregate().

So you really should not use .aggregate() for this at all.