M101J: MongoDB for Java Developers Homework 5.1

Question: Finding the most frequent author of comments on your blog.

 Solution:

you need to use webshell to find the most frequent author of comments 

Step 1:

Understand Structure of posts collection

{
    "_id" : ObjectId("540d427e132c1f13547188cc"),
    "body" : "empty_post",
    "permalink" : "cxzdzjkztkqraoqlgcru",
    "author" : "machine",
    "title" : "US Constitution",
    "tags" : [
        "january",
        "mine",
        "modem",
        "literature",
        "saudi arabia",
        "rate",
        "package",
        "respect",
        "bike",
        "cheetah"
    ],
    "comments" : [
        {
            "body" : "empty_comment",
            "email" : "eAYtQPfz@kVZCJnev.com",
            "author" : "Kayce Kenyon"
        },.........

 

2) we need to count comments so we will unwind the comments first using

 {
        $unwind: "$comments"
    }

 

3) then we need to group comment as per author so we will add group query with it count using sum.

{

$group: {

"_id": "$comments.author",

"num_comments": {

$sum: 1

}

}

}

 4) then we will sort from max to min number of comments. so we will add 

{
        $sort: {
            "num_comments": 1
        }
    }

5) note: this is large data so we will add limit to 1 rows by using
{$limit: 1}

so your final query will be

db.posts.aggregate([

{

$project: {

"_id": 0,

"comments": 1

}

},

{

$unwind: "$comments"

},

{

$group: {

"_id": "$comments.author",

"num_comments": {

$sum: 1

}

}

},

{

$sort: {

"num_comments": 1

}

},

{

$limit: 1

}

]

};

AND ANSWER I GOT IS Gisela Levin