How rename nested key in array of object in MongoDB?

Question

Document Structure

{
  _id: 5,
  grades: [
     { grade_ : 80, mean: 75, std: 8 },
     { mean: 90, std: 5 },
     { mean: 85, std: 3 }
  ]
}

As per above document structure in mongodb i want rename key grade_ to grade

db.collection.update({"_id":5},{"$rename":{"grades.grade_":"grades.grade"}},{"upsert":false,"multi":true})

which gives below error

"writeError" : {
        "code" : 28,
        "errmsg" : "cannot use the part (grades of grades.grade_) to traverse the element ({grades: [ { grade_: 80.0, mean: 75.0, std: 8.0 }, { mean: 90.0, std: 5.0 }, { mean: 85.0, std: 3.0 } ]})"
    }

I want to rename key grade_ to grade, expected output

{
  _id: 5,
  grades: [
     { grade : 80, mean: 75, std: 8 },
     { mean: 90, std: 5 },
     { mean: 85, std: 3 }
  ]
}

Hardik Shah · Accepted Answer · 2021-11-17 14:46:37Z

4

As per MongoDB documentation: ($rename does not work if these fields are in array elements.)

For fields in embedded documents, the $rename operator can rename these fields as well as move the fields in and out of embedded documents. $rename does not work if these fields are in array elements.

So, you need to write your custom logic to update.

db.collection.find({
  "grades.grade_": { $exists : 1 }
}).forEach( function( doc ) {
  for( i=0; i < doc.grades.length; i++ ) {
    if(doc.grades[i].grade_ != undefined) {
      doc.grades[i].grade = doc.grades[i].grade_;
      delete doc.grades[i].grade_;
    }
  }
  db.collection.update({ _id : doc._id }, doc);
});

edited Nov 17, 2021 at 14:46

answered Jul 30, 2019 at 9:14

Hardik Shah

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2 Comments

Jacques Over a year ago

Is the space between grades and grade_ intentional? Line two of your example

Hardik Shah Over a year ago

@Jacques Must be some typo, Thank you to point out. But, it will not affect the end result

MilanRegmi · Accepted Answer · 2019-07-30 10:15:59Z

4

$rename do not works in an array. So,you can use Aggregate framework's $addField to rename fields in an array.

db.collection.aggregate([
  {
    $addFields: {
      grades: {
        $map: {
          input: "$grades",
          as: "grade",
          in: {
            grade: "$$grade.grade_",
            mean: "$$grade.mean",
            std: "$$grade.std"
          }
        }
      }
    }
  }
])

Output:

[
  {
    "_id": 5,
    "grades": [
      {"grade": 80,"mean": 75,"std": 8},
      {"mean": 90,"std": 5},
      {"mean": 85,"std": 3}
    ]
  }
]

edited Jul 30, 2019 at 10:15

answered Jul 30, 2019 at 9:56

MilanRegmi

5193 silver badges12 bronze badges

3 Comments

MilanRegmi Over a year ago

Clink the link and run it, if you want to test it by yourself. mongoplayground.net/p/8FbX6sBrhA3

Krunal Sonparate Over a year ago

This solution will not work for me, as it will require to add all the possible keys in $map.in object. In my case there can be more than 400 possible keys

user09 Over a year ago

@MilanRegmi I am trying to do similar aggregation and having issues. can you help me here stackoverflow.com/questions/61873974/…

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