How rename nested key in array of object in MongoDB? - 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 }
]
}

Answer 1

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);
});

Answer 2

$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}
]
}
]