Mongodb group documents and limit each group

Question

Considering those documents:

{
    "Field1":"Test_1",
    "Speaker":1,
    "Listener":2,
    "ListenTime": ISODate("2016-10-15T14:17:49.336Z")
},
{
    "Field1":"Test_2",
    "Speaker":1,
    "Listener":2,
    "ListenTime": ISODate("2016-10-13T14:17:49.336Z")
},
{
    "Field1":"Test_3",
    "Speaker":1,
    "Listener":3,
    "ListenTime": ISODate("2016-10-10T14:10:49.336Z")
}

What i what to do is to extract with a single query in MongoDB (3.2) only those documents grouping them by Speaker and Listener, taking only the document with the oldest ListenTime.
So, in this case, the result will be:

{
    "Field1":"Test_1",
    "Speaker":1,
    "Listener":2,
    "ListenTime": ISODate("2016-10-15T14:17:49.336Z")
},
{
    "Field1":"Test_3",
    "Speaker":1,
    "Listener":3,
    "ListenTime": ISODate("2016-10-10T14:10:49.336Z")
}

Is it possible do that with a single query?

Answer 1

Run the following aggregation pipeline to get the desired result:

db.collection.aggregate([
    { "$sort": { "ListenTime": -1 } },
    {
        "$group": {
            "_id": {
                "Speaker": "$Speaker",
                "Listener": "$Listener"
            },
            "Field1" : { "$first": "$Field1" },
            "ListenTime" : { "$first": "$ListenTime" }
        }
    },
    {
        "$project": {
            "Field1": 1,
            "Speaker": "$_id.Speaker",
            "Listener": "$_id.Listener",
            "ListenTime": 1,
            "_id": 0
        }
    }
])

Sample Output

/* 1 */
{
    "Field1" : "Test_3",
    "ListenTime" : ISODate("2016-10-10T14:10:49.336Z"),
    "Speaker" : 1,
    "Listener" : 3
}

/* 2 */
{
    "Field1" : "Test_1",
    "ListenTime" : ISODate("2016-10-15T14:17:49.336Z"),
    "Speaker" : 1,
    "Listener" : 2
}