Mongodb Aggregation count array/set size

Question

Here\'s my problem:

Model:

{ application: \"abc\", date: Time.now, status: \"1\" user_id: [ id1, id2, id4] }

{ application: \"abc\",

Answer 1

Use $size to get the size of set.

[
    {
        $match: {"application": "abc"}
    },
    {
        $unwind: "$user_id"
    },
    {
        $group: {
            "_id": "$status",
            "application": "$application",
            "unique_user_id": {$addToSet: "$user_id"}
        }
    },
    {
        $project:{
            "_id": "$_id",
            "application": "$application",
            "count": {$size: "$unique_user_id"}
        }
    }
]