计算值与数组不匹配的地方 [英] Count where value does not match array

问题描述

我有以下offers集合

此处readBy包含用户的_id ... 现在，我要计算userId = "5add82620d7f5b38240c63d4"

Here readBy contains the _id of the users... Now I want to count the number of unRead offers for the userId = "5add82620d7f5b38240c63d4"

{
    "_id" : ObjectId("5aeaab5ed6a9c97d0209260a"),
    "expiresIn" : ISODate("2018-05-30T18:30:00.000Z"),
    "name" : "Trip ",
    "readBy" : [ 
        ObjectId("5add82620d7f5b38240c63d4"),
        ObjectId("5add82620d7f5b38240c63c6")
    ],
    "__v" : 0
}
{
    "_id" : ObjectId("5aeaab7dd6a9c97d0209260b"),
    "expiresIn" : ISODate("2018-05-29T18:30:00.000Z"),
    "name" : "Trip",
    "readBy" : [ObjectId("5add82620d7f5b38240c63d4")],
    "__v" : 0
}
{
    "_id" : ObjectId("5aeae233d6a9c97d02092622"),
    "expiresIn" : ISODate("2018-05-25T18:30:00.000Z"),
    "name" : "two way off",
    "readBy" : [],
}
{
    "_id" : ObjectId("5aeae49643f10d284726069c"),
    "expiresIn" : ISODate("2018-05-25T18:30:00.000Z"),
    "name" : "two way off",
    "readBy" : [],
}
{
    "_id" : ObjectId("5aeae49743f10d284726069d"),
    "expiresIn" : ISODate("2018-05-25T18:30:00.000Z"),
    "name" : "two way off",
    "readBy" : []
}
{
    "_id" : ObjectId("5aeae49743f10d284726069e"),
    "expiresIn" : ISODate("2018-05-25T18:30:00.000Z"),
    "name" : "two way off",
    "readBy" : []
}

因此对于上述集合，我的输出应为

so for the above collection my output should be

[{
  numberOfUnreadOffers: 4
}]

因为集合中有四个在readBy数组中没有5add82620d7f5b38240c63d4

because four of the collection does not have 5add82620d7f5b38240c63d4 in readBy array

推荐答案

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.

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