Perform $lookup to array of object id's in MongoDB?

To perform $lookup on an array of ObjectIds in MongoDB, use the $lookup aggregation stage with a pipeline approach. This performs a left outer join between collections where one collection contains an array of ObjectIds referencing documents in another collection.

Syntax

db.collection.aggregate([
    {
        $lookup: {
            from: "targetCollection",
            let: { arrayField: "$arrayOfObjectIds" },
            pipeline: [
                { $match: { $expr: { $in: ["$_id", "$$arrayField"] } } }
            ],
            as: "joinedData"
        }
    }
])

Create Sample Data

First, create a collection with user documents ?

db.users.insertMany([
    { Name: "Chris" },
    { Name: "David" }
]);

{
    "acknowledged": true,
    "insertedIds": {
        "0": ObjectId("5e5e782317aa3ef9ab8ab207"),
        "1": ObjectId("5e5e782317aa3ef9ab8ab208")
    }
}

Next, create a collection containing an array of ObjectIds referencing the users ?

db.groups.insertOne({
    "details": [
        ObjectId("5e5e782317aa3ef9ab8ab207"),
        ObjectId("5e5e782317aa3ef9ab8ab208")
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e5e787817aa3ef9ab8ab209")
}

Perform $lookup with Array of ObjectIds

db.groups.aggregate([
    {
        $lookup: {
            from: "users",
            let: { details: "$details" },
            pipeline: [
                { $match: { $expr: { $in: ["$_id", "$$details"] } } }
            ],
            as: "matchedUsers"
        }
    }
]);

{
    "_id": ObjectId("5e5e787817aa3ef9ab8ab209"),
    "details": [
        ObjectId("5e5e782317aa3ef9ab8ab207"),
        ObjectId("5e5e782317aa3ef9ab8ab208")
    ],
    "matchedUsers": [
        { "_id": ObjectId("5e5e782317aa3ef9ab8ab207"), "Name": "Chris" },
        { "_id": ObjectId("5e5e782317aa3ef9ab8ab208"), "Name": "David" }
    ]
}

How It Works

• $let defines a variable details containing the array of ObjectIds
• $match with $expr checks if each document's _id exists in the array using $in
• $$details references the variable defined in the let stage
• The as field specifies the output array name for joined documents

Conclusion

Use $lookup with a pipeline approach to join collections when one contains an array of ObjectIds. The $in operator within $expr efficiently matches documents whose _id exists in the reference array.