Mongo Output for lookup query

output.bson

{ 
    "_id" : ObjectId("5a36ce46f7a13634daf1214c"), 
    "name" : "Jayden", 
    "created_at" : ISODate("2017-12-14T12:49:01.746+0000"), 
    "subscriptions" : [
        {
            "_id" : ObjectId("5a36ce84f7a13634daf1214e"), 
            "user_id" : ObjectId("5a36ce46f7a13634daf1214c"), 
            "topic" : "nodejs"
        }, 
        {
            "_id" : ObjectId("5a36ceaaf7a13634daf12150"), 
            "user_id" : ObjectId("5a36ce46f7a13634daf1214c"), 
            "topic" : "mongodb"
        }
    ]
}
{ 
    "_id" : ObjectId("5a36cec2f7a13634daf12152"), 
    "name" : "Olivia", 
    "created_at" : ISODate("2017-12-15T12:49:01.746+0000"), 
    "subscriptions" : [
        {
            "_id" : ObjectId("5a36cec2f7a13634daf12152"), 
            "user_id" : ObjectId("5a36cec2f7a13634daf12152"), 
            "topic" : "java"
        }
    ]
}