MongoDB | get max value from nested object

Question

db.getCollection('post').find({'post_no':47}, {'comment': 1})

The resulting values are:

{
    "_id" : ObjectId("5bc05c038e798ccb0309658b"),
    "comment" : [ 
        {
            "comment_no" : 112
        }, 
        {
            "comment_no" : 113,
            "comment_group" : 1
        }, 
        {
            "comment_no" : 116,
            "comment_group" : 2
        }, 
        {
            "comment_no" : 117,
            "comment_group" : 3
        }, 
        {
            "comment_group" : 4,
            "comment_no" : 118
        }
    ]
}

I want to get the maximum value 4 of the comment_group.

What can I do?

Thank you for your advice.

Rubin Porwal · Accepted Answer · 2018-10-12 12:59:36Z

1

db.collection.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $unwind: {
                path: "$comment",

            }
        },

        // Stage 2
        {
            $sort: {
                "comment.comment_group": -1
            }
        },

        // Stage 3
        {
            $limit: 1
        },

    ]



);

answered Oct 12, 2018 at 12:59

Rubin Porwal

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Comments

Ashh · Accepted Answer · 2018-10-15 02:49:22Z

1

You can try below aggregation

db.collection.aggregate([
  { "$project": {
    "comment": {
      "$map": {
        "input": "$comment",
        "in": {
          "comment_no": "$$this.comment_no",
          "comment_group": { "$ifNull": [ "$$this.comment_group", 0 ] }
        }
      }
    }
  }},
  { "$project": {
    "comment": {
      "$arrayElemAt": [
        "$comment",
        {
          "$indexOfArray": [
            "$comment.comment_group",
            { "$max": "$comment.comment_group" }
          ]
        }
      ]
    }
  }}
])

edited Oct 15, 2018 at 2:49

answered Oct 12, 2018 at 9:50

Ashh

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8 Comments

fly high Over a year ago

Thank you!! I changed it from 'comment_no' to 'comment_group' and applied it. But The result is incorrect. The maximum value is 3, not 4. It seems that the comment_group value does not exist in all objects. How can I supplement this?

Ashh Over a year ago

Updated my answer

fly high Over a year ago

The third method works. It was very helpful. Thank you!!

Ashh Over a year ago

@flyhigh Well Accepted answer won't give you the performance.

fly high Over a year ago

Do your answers show better performance? Could you explain more?

|

Connor · Accepted Answer · 2019-05-09 08:45:20Z

0

You can alse use $reduce in the second project in Anthony Winzlet's answer.

{
  "$project": {
    "comment": {
      '$reduce': {
        'input': '$comment',
        'initialValue': {'$arrayElemAt': ['$comment', 0]},
        'in': {
          '$cond': {
            'if': {'$gt': ['$$this.comment_group', '$$value.comment_group']},
            'then': '$$this',
            'else': '$$value'
          }
        }
      }
    }
  }
}

answered May 9, 2019 at 8:45

Connor

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