Reverse array field in MongoDB?

To reverse array field in MongoDB, you can use the forEach() method to iterate through documents and manually reorder array elements using the $set operator.

Syntax

db.collection.find().forEach(function (document) {
    var reversedArray = document.arrayField.reverse();
    db.collection.update(document, { $set: {arrayField: reversedArray } });
});

Sample Data

Let us first create a collection with documents ?

db.reverseArrayDemo.insertOne({"Skills":["C","Java"]});

{
    "acknowledged" : true,
    "insertedId" : ObjectId("5ccddf99dceb9a92e6aa1946")
}

Following is the query to display all documents from the collection ?

db.reverseArrayDemo.find().pretty();

{
    "_id" : ObjectId("5ccddf99dceb9a92e6aa1946"),
    "Skills" : [
        "C",
        "Java"
    ]
}

Method 1: Using forEach() with Manual Index Reversal

Here is the query to reverse array field in MongoDB ?

db.reverseArrayDemo.find().forEach(function (myDocument) {
    var arrayValue = [ myDocument.Skills[1], myDocument.Skills[0] ];
    db.reverseArrayDemo.update(myDocument, { $set: {Skills : arrayValue } });
});

Verify Result

Let us display the document from the collection to check if the array field is now reversed ?

db.reverseArrayDemo.find().pretty();

{
    "_id" : ObjectId("5ccddf99dceb9a92e6aa1946"),
    "Skills" : [
        "Java",
        "C"
    ]
}

Method 2: Using JavaScript reverse() Function

For arrays with dynamic length, use the JavaScript reverse() method ?

db.reverseArrayDemo.find().forEach(function (doc) {
    var reversedSkills = doc.Skills.reverse();
    db.reverseArrayDemo.update({_id: doc._id}, { $set: {Skills: reversedSkills } });
});

Key Points

• forEach() iterates through each document in the collection.
• Manual indexing works for fixed-size arrays, while reverse() handles dynamic arrays.
• Use $set operator to update the array field with reversed values.

Conclusion

MongoDB doesn't have a built-in reverse operator, so use forEach() with JavaScript array methods or manual indexing to reverse array fields. The reverse() method is recommended for dynamic-length arrays.