Access Deeply Nested Array of Subdocuments in Mongodb

November 23, 2018

Example mongodb document

    {
        _id: ObjectID("4b1bb6ff3abc8266a21890co")
        listing: {
            name: "foo"
            address: "ham street"
        },
        categories: [
            {
                title: "bar"
                items: [
                    {
                        name: "eggs"
                    }
                ]
            },
            {
                title: "baz"
                items: [
                    {
                        name: "french toast"
                    }
                ]
            }
        ]
    }

Use mongodb aggregation to access categories -> items -> name and return the embedded document

assuming our database has a collection named listings and looking for item names that have the word french

db.listings.aggregate([
    
    /* Make $match & $sort first in aggregation pipelines to take advantage of indexes */
    { $match: {"categories.items.name": {$regex: "french", $options: "i"}}},
    { $unwind: "$categories" },
    { $unwind: "$categories.items" },
    { $limit: 5 },
    { $project: {_id: 0, item: { name: "$categories.items.name"}}}

])

$unwind both arrays categories and items which essentialy creates new documents.

From there, $project any property of the document.

{
    item: {
        name: "french toast"
    }
}