See http://stackoverflow.com/a/8746805/483520

users_comments.js

/* 1 */
{
    "_id" : ObjectId("57eaf642ff01db2659e2af49"),
    "value" : {
        "country" : "US",
        "gender" : "F",
        "age" : "13"
    }
}

/* 2 */
{
    "_id" : ObjectId("57eaf642ff01db2659e2af47"),
    "value" : {
        "comments" : [ 
            {
                "commentId" : ObjectId("57eaf642ff01db2659e2af4a"),
                "comment" : "Hey, what's up?",
                "created" : ISODate("2016-09-27T22:44:18.195Z")
            }, 
            {
                "commentId" : ObjectId("57eaf642ff01db2659e2af4c"),
                "comment" : "Cool",
                "created" : ISODate("2016-09-27T22:44:18.212Z")
            }
        ],
        "country" : "CA",
        "gender" : "M",
        "age" : "18"
    }
}

/* 3 */
{
    "_id" : ObjectId("57eaf642ff01db2659e2af48"),
    "value" : {
        "comments" : [ 
            {
                "commentId" : ObjectId("57eaf642ff01db2659e2af4b"),
                "comment" : "Not much",
                "created" : ISODate("2016-09-27T22:44:18.211Z")
            }
        ],
        "country" : "US",
        "gender" : "M",
        "age" : "25"
    }
}