How to append to array in MongoDB?

To append to array in MongoDB, use the $concatArrays operator in aggregation pipelines to combine multiple arrays into a single array. This operator concatenates arrays and returns the combined result.

Syntax

db.collection.aggregate([
    {
        $project: {
            newField: {
                $concatArrays: ["$array1", "$array2", ...]
            }
        }
    }
])

Sample Data

db.demo435.insertMany([
    {"FirstName": ["Chris"], "LastName": ["Brown"]},
    {"FirstName": ["David"], "LastName": ["Miller"]},
    {"FirstName": ["John"], "LastName": ["Doe"]}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e7719b1bbc41e36cc3cae97"),
        ObjectId("5e7719bdbbc41e36cc3cae98"),
        ObjectId("5e7719c6bbc41e36cc3cae99")
    ]
}

Display Sample Data

db.demo435.find().pretty();

{
    "_id": ObjectId("5e7719b1bbc41e36cc3cae97"),
    "FirstName": ["Chris"],
    "LastName": ["Brown"]
}
{
    "_id": ObjectId("5e7719bdbbc41e36cc3cae98"),
    "FirstName": ["David"],
    "LastName": ["Miller"]
}
{
    "_id": ObjectId("5e7719c6bbc41e36cc3cae99"),
    "FirstName": ["John"],
    "LastName": ["Doe"]
}

Example: Concatenate FirstName and LastName Arrays

Append the LastName array to the FirstName array to create a combined FullName array ?

db.demo435.aggregate([
    {
        $project: {
            FullName: {
                $concatArrays: ["$FirstName", "$LastName"]
            }
        }
    }
]);

{"_id": ObjectId("5e7719b1bbc41e36cc3cae97"), "FullName": ["Chris", "Brown"]}
{"_id": ObjectId("5e7719bdbbc41e36cc3cae98"), "FullName": ["David", "Miller"]}
{"_id": ObjectId("5e7719c6bbc41e36cc3cae99"), "FullName": ["John", "Doe"]}

Key Points

• $concatArrays works only in aggregation pipelines, not in update operations.
• The operator preserves the order of elements from left to right.
• If any input array is null, the result will be null.

Conclusion

The $concatArrays operator efficiently combines multiple arrays into one during aggregation. It maintains element order and is ideal for creating composite arrays from existing array fields.