MongoDB query to replace value in an array?

To replace a specific value in a MongoDB array, use the $set operator combined with the $ positional operator. The positional operator identifies the array element that matches the query condition and allows you to update it.

Syntax

db.collection.update(
    { "arrayField": "valueToReplace" },
    { $set: { "arrayField.$": "newValue" } }
);

Sample Data

db.replaceValueInArrayDemo.insertMany([
    { "StudentScores": [45, 56, 78] },
    { "StudentScores": [33, 90, 67] }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd7f0421a844af18acdffb7"),
        ObjectId("5cd7f0521a844af18acdffb8")
    ]
}

Display all documents to verify the data ?

db.replaceValueInArrayDemo.find().pretty();

{
    "_id": ObjectId("5cd7f0421a844af18acdffb7"),
    "StudentScores": [ 45, 56, 78 ]
}
{
    "_id": ObjectId("5cd7f0521a844af18acdffb8"),
    "StudentScores": [ 33, 90, 67 ]
}

Example: Replace Value 45 with 99

db.replaceValueInArrayDemo.update(
    { _id: ObjectId("5cd7f0421a844af18acdffb7"), StudentScores: 45 },
    { $set: { 'StudentScores.$': 99 } }
);

WriteResult({ "nMatched": 1, "nUpserted": 0, "nModified": 1 })

Verify Result

db.replaceValueInArrayDemo.find().pretty();

{
    "_id": ObjectId("5cd7f0421a844af18acdffb7"),
    "StudentScores": [ 99, 56, 78 ]
}
{
    "_id": ObjectId("5cd7f0521a844af18acdffb8"),
    "StudentScores": [ 33, 90, 67 ]
}

Key Points

• The $ operator identifies the first matching array element in the query condition.
• Include the target value in both the query condition and the array field to ensure precise matching.
• This method only updates the first occurrence of the matching value.

Conclusion

Use $set with the $ positional operator to replace specific values in MongoDB arrays. The query condition must include the target value to identify which array element to update.