How to group by counted elements using aggregation in MongoDB?

I have to write some aggregation using MQL. The case is:

1. There is a chat application
2. Collection contains info about: thread, sent message, sent time, read time

Sample records are shown below.

message_collection

[
    {
      "thread" : "war conference",
      "message" : "stop",
      "sentInMilli": 1696858510000, // oldest message in war conference
      "statuses": [ {"readInMilli": 1696858547857} ]
    },
    {
      "thread" : "economic conference",
      "message" : "welcome on economic conference",
      "sentInMilli": 1696858520000, // newest message in economic conference
      "statuses": [ {"readInMilli": 1696858547857} ]
    },
    {
      "thread" : "war conference",
      "message" : "the war",
      "sentInMilli": 1696858530000, // newest message in war conference
      "statuses": [ ]
    }
]

What I want to do is to display data about threads. To be more specific I need to know which thread have unread messages. As you can see in the example above, there are 2 threads: war conference and economic conference.

So after my supposable aggregation I want to get that kind of records:

[
  {
    "thread": "war conference",
    "timeInMilliOfNewestMessage": 1696858530000,
    "containsUnreadMessages": true
  },
  {
    "thread": "economic conference",
    "timeInMilliOfNewestMessage": 1696858520000,
    "containsUnreadMessages": false
  }
]

My current pseudo-query:

db.getCollection('message_collection').aggregate(
    [
        {
            "$match": { } // my other conditions...
        },
        {
            "$group": {
                "_id": "thread", 
                "timeInMilliOfNewestMessage": {"$max": "$sentInMilli"},
                "containsUnreadMessages": { 
                    "$count": {
                        "statuses": {
                            "$elemMatch": {
                                "readInMilli": { "$exists": false }
                            } 
                        }
                    }
                }
            }
        }
    ]
)

As you can see, I have an idea to firstly count all messages that are already read. Then I would like to convert it to boolean ones.

I got this error:

Unrecognized expression '$elemMatch'

I know this syntax is invalid but I don’t know why :/
Maybe what I want to achieve is impossible in the group section?
Could someone help me with that?

db.version()
4.0.0

>Solution :

As you are using MongoDB v4.0+, one of the possible solutions is to $min the array size of statuses and check for whether it is 0 or not

db.collection.aggregate([
  {
    "$group": {
      "_id": "$thread",
      "timeInMilliOfNewestMessage": {
        "$max": "$sentInMilli"
      },
      "containsUnreadMessages": {
        "$min": {
          $size: "$statuses"
        }
      }
    }
  },
  {
    "$addFields": {
      "containsUnreadMessages": {
        "$cond": {
          "if": {
            $eq: [
              "$containsUnreadMessages",
              0
            ]
          },
          "then": true,
          "else": false
        }
      }
    }
  }
])

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