MongoD size of multiple nested arrays

Question

I have collection of poems. The document in the collection has the following structure:

{
"_id" : "Romeo and Juliet",
"acts" : [ 
    {
        "title" : "ACT I",
        "scenes" : [ 
            {
                "title" : "SCENE I. Verona. A public place.",
                "action" : [ 
                    {
                        "character" : "SAMPSON",
                        "says" : [ 
                            "Gregory, o' my word, we'll not carry coals."
                        ]
                    }, 
                    {
                        "character" : "GREGORY",
                        "says" : [ 
                            "No, for then we should be colliers."
                        ]
                    }, 
                    // ...
                    {
                        "character" : "GREGORY",
                        "says" : [ 
                            "To move is to stir; and to be valiant is to stand:", 
                            "therefore, if thou art moved, thou runn'st away."
                        ]
                    }, 
                    {
                        "character" : "SAMPSON",
                        "says" : [ 
                            "A dog of that house shall move me to stand: I will", 
                            "take the wall of any man or maid of Montague's."
                        ]
                    }, 
                    {
                        "character" : "GREGORY",
                        "says" : [ 
                            "That shows thee a weak slave; for the weakest goes", 
                            "to the wall."
                        ]
                    }
                    // ...
                ]
            }
            // ...
        ]
    }
    // ...
]}

I want to count number of acts and scenes for each poem. I tried to do something like this, but the result is incorrect.

db.poems.aggregate([{$unwind:"$acts"}, {$unwind:"$acts.scenes"}, {$group: {_id: "$_id", pa: {$push: {poemAct:"$acts"}}, ps: {$push: {poemScenes:"$acts.scenes"}}}}, {$project: {numberOfActs: {$size: "$pa"}, numberOfScenes: {$size: "$ps"}}}]).pretty()

Help somebody, please :D

what is the format of expected output?

Atish
– Atish

2018-02-10 20:16:53 +00:00
Commented Feb 10, 2018 at 20:16 — Atish
– Atish, Commented Feb 10, 2018 at 20:16

s7vr · Accepted Answer · 2018-02-10 21:58:13Z

8

You can try below aggregation. No $unwind required here.

$map and $size to output array with scenes sizes and $sum to count the array values.

db.poems.aggregate({
  "$project":{
    "numberOfActs":{"$size":"$acts"},
    "numberOfScenes":{
      "$sum":{
        "$map":{
          "input":"$acts",
          "in":{"$size":"$$this.scenes"}
        }
      }
    }
  }
})

answered Feb 10, 2018 at 21:58

s7vr

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Atish · Accepted Answer · 2018-02-10 20:25:07Z

2

db.poems.find().forEach(function(poem) {
    var res = {};
    res._id = poem._id, 
    res.acts = poem.acts.length;
    res.scenes = poem.acts.scenes.length;
    printjson(res)
})

answered Feb 10, 2018 at 20:25

Atish

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