How to get the intersection of two arrays in MongoDB?

To get intersection of two arrays in MongoDB, use $setIntersection along with aggregate(). This operator returns an array containing elements that appear in both input arrays.

Syntax

db.collection.aggregate([
    {
        $project: {
            fieldName: { $setIntersection: ["$array1", "$array2"] }
        }
    }
]);

Sample Data

Let us create a collection with documents ?

db.demo61.insertOne({
    "Values1": [10, 20, 30, 40, 50],
    "Values2": [30, 100, 70, 120, 40]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e286e28cfb11e5c34d8992a")
}

Display all documents from a collection with the help of find() method ?

db.demo61.find().pretty();

{
    "_id": ObjectId("5e286e28cfb11e5c34d8992a"),
    "Values1": [
        10,
        20,
        30,
        40,
        50
    ],
    "Values2": [
        30,
        100,
        70,
        120,
        40
    ]
}

Example

Following is the query to get the intersection of two arrays in MongoDB ?

db.demo61.aggregate([
    {
        $project: {
            BothValues: { $setIntersection: ["$Values1", "$Values2"] }
        }
    }
]);

{ "_id": ObjectId("5e286e28cfb11e5c34d8992a"), "BothValues": [30, 40] }

Key Points

• $setIntersection removes duplicates and returns unique elements present in both arrays.
• The order of elements in the result may not match the original arrays.
• Works with arrays containing any BSON data types.

Conclusion

Use $setIntersection with the aggregation framework to find common elements between two arrays. This operator efficiently returns the intersection as a new array field in your projection.