🦸 Missing Data

Sometimes, we're missing a field we're looking for (it's not there, as documents in a collection can be polymorphic, so this field is null). For these cases, we can check if something is there using $ifNull and add a default value.

Atlas UI

[
    {
        $project: {
            name: 1,
            books: { $ifNull: ["$books", []]},
        }
    }
]

MongoDB Shell

let getAuthorsWithBooks = {
    $project: {
        name: 1,
        books: { $ifNull: ["$books", []]},
    }
}

db.authors.aggregate([
    getAuthorsWithBooks,
])