How do you remove an array element by its index in MongoDB

To remove an array element by its index in MongoDB, you can use the $unset and $pull operators in a two-step process. The $unset operator sets the array element at the specified index to null, and the $pull operator removes all null values from the array.

Syntax

db.yourCollectionName.update({}, {$unset: {"yourArrayListName.yourPosition": yourPositionValue}});
db.yourCollectionName.update({}, {$pull: {"yourArrayListName": null}});

Sample Data

Let us create a collection with a document to demonstrate the removal process ?

db.removeArrayElements.insertOne({
    "StudentName": "Larry",
    "StudentAge": 23,
    "TechnicalSubject": ["C", "C++", "Java", "MongoDB"]
});

{
    "acknowledged" : true,
    "insertedId" : ObjectId("5c6ea4879c4643706aef56d2")
}

Display the document using the find() method ?

db.removeArrayElements.find().pretty();

{
    "_id" : ObjectId("5c6ea4879c4643706aef56d2"),
    "StudentName" : "Larry",
    "StudentAge" : 23,
    "TechnicalSubject" : [
        "C",
        "C++",
        "Java",
        "MongoDB"
    ]
}

Example: Remove Element at Index 3

Let us remove the "MongoDB" element, which is at index 3 (position 4) in the TechnicalSubject array ?

db.removeArrayElements.update({}, {$unset: {"TechnicalSubject.3": 1}});
db.removeArrayElements.update({}, {$pull: {"TechnicalSubject": null}});

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

Verify Result

Let us display the updated document to confirm the element was removed ?

db.removeArrayElements.find().pretty();

{
    "_id" : ObjectId("5c6ea4879c4643706aef56d2"),
    "StudentName" : "Larry",
    "StudentAge" : 23,
    "TechnicalSubject" : [
        "C",
        "C++",
        "Java"
    ]
}

As you can see in the output, the element "MongoDB" at index 3 has been successfully removed from the TechnicalSubject array.

How It Works

• Step 1: $unset sets the element at the specified index to null, creating a gap in the array
• Step 2: $pull removes all null values from the array, effectively closing the gap
• Array indexing is zero-based, so index 3 refers to the fourth element

Conclusion

Removing array elements by index in MongoDB requires a two-step process using $unset to nullify the element and $pull to remove null values. This approach maintains array structure while successfully removing elements at specific positions.