How to access data from nested arrays in mongodb

Question

I am completely new to mongodb and I was trying to get an object from nested array, in the above mentioned array of objects I want to access an object where Fname should be Svelte and chapterName should be Chapter 0.

    [
      {
        _id: { oid: '6393fc35a2bd7da5c33e31a9' },
        Fname: 'Svelte',
        __v: { numberInt: '0' },
        data: [
          {
            chapterName: 'Chapter 0',
            notes: [
              [
                '<h2 id="heading1">About Svelte</h2>',
                '<br>',
                '<p id="p1">Its an compiler which compiles all the code and then bundel it using any one of the bundling tools.</p>',
                '<br>',
                '<p id="p2">It is very easy to understand as it somewhat looks like vanilla javascript.</p>'
              ]
            ],
            _id: { '$oid': '6393fc35a7f6aef6477ab3eb' }
          },
          {
            chapterName: 'Chapter 1',
            notes: [
              [
                '<h2 id="heading1">onMount</h2>',
                '<br>',
                '<p id="p1">something about onMount funciton in svelte.</p>'
              ]
            ],
            _id: { '$oid': '639425eb9037e86ffc337505' }
          }
        ]
      },
      {
        _id: { oid: '6395718e137c1a3763a0fefd' },
        Fname: 'Cpp',
        __v: { '$numberInt': '0' },
        data: [
          {
            chapterName: 'Chapter 0',
            notes: [
              [
                '<br>',
                '<p id="p1">Enter your paragraph here.</p>',
                '<h2 id="heading2">Enter your heading here</h2>'
              ]
            ],
            _id: { '$oid': '6395718e15ec262729cc45be' }
          }
        ]
      }
    ]

In short, I need this object

{
            chapterName: 'Chapter 0',
            notes: [
              [
                '<h2 id="heading1">About Svelte</h2>',
                '<br>',
                '<p id="p1">Its an compiler which compiles all the code and then bundel it using any one of the bundling tools.</p>',
                '<br>',
                '<p id="p2">It is very easy to understand as it somewhat looks like vanilla javascript.</p>'
              ]
            ],
            _id: { '$oid': '6393fc35a7f6aef6477ab3eb' }
          }

What will be the query to extract this object ? Thank you so much for you help.

R2D2 · Accepted Answer · 2022-12-11 12:00:42Z

Option 1:

db.collection.find({
  "data.chapterName": "Chapter 1",
   Fname: "Svelte"
},
{
  "data.$": 1
})

Explained:

Using find/project you can filter first found object in the data array. ( Please, note if there is second object in same array mathing the criteria it will not be shown )

Playground

Option 2

db.collection.aggregate([
{
  $match: {
  "data.chapterName": "Chapter 1",
  Fname: "Svelte"
 }
},
{
 "$addFields": {
  "data": {
    "$filter": {
      "input": "$data",
      "as": "d",
      "cond": {
        "$eq": [
          "$$d.chapterName",
          "Chapter 1"
        ]
      }
    }
    }
   }
   },
   {
   $unwind: "$data"
   },
  {
  "$replaceRoot": {
    "newRoot": "$data"
   }
  }
])

Eplained:

1).Using aggregate/filter you can filter all objects matching the criteria under data array 2).Using unwind/replaceRoot you can flatten the array and have only the matching objects in final result.

Playground2

P.S. If you dont have already , having compount index on below fields will improve performance of your query:

 db.collection.createIndex("data.ChapterName":1,"Fname":1})

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