Update or Insert new data in nested array MongoDB

Question

I'm using Node JS with Mongoose fyi. Given I've something like this:-

let people = [
    {
        "_id": 1,
        "name": "Person 1",
        "pets": [
            {
                "_id": 1,
                "name": "Tom",
                "category": "cat",
                "favFood": [
                    {
                        "_id": 1,
                        "name": "steamed fish"
                    }
                ]
            },
            {
                "_id": 2,
                "name": "Jerry",
                "category": "mouse",
                "favFood": [
                    {
                        "_id": 1,
                        "name": "cheese"
                    }
                ] 
            }
        ]
    }
]

Right now I know how to insert new pet data like shown below :-

// new Pet Data to INSERT
let newPetData = {
    name: "Butch",
    category: "dog",
    favFood: [
        {
            name: "bone"
        }
    ]    
}

// INSERT new data in DB
People.updateOne(
    // push into the sub-documents - pets
    { $push: { pets: newPetData } },
    (err, success) => {
        if(err) res.json({ message: `Unsuccessful!`, report: err })
        else res.json({ message: `Successful!`, report: success })
    }
)

But how can I insert new favFood data of a pet if I have their id (id of the pet) ?

let petID = 1 // this is the ID of pet named Tom

// this is a new favorite food of Tom to INSERT
let newFavFood = {
    name: "fried fish"
}

// code to INSERT new favorite food of Tom (THIS DOES NOT WORK!)
People.updateOne(
    { "_id": petID } // id of pet 'Tom'
    { $push: { favFood: newFavFood } }, // push into the sub-documents - favFood
    (err, success) => {
        if(err) res.json({ message: `Unsuccessful!`, report: err })
        else res.json({ message: `Successful!`, report: success })
    }
)    

The above code does not work. I thought that if I specify the id of a pet like show above { "_id": petID } would yield what I want. Even though it returns a message of Successful! . But still no new record added to the db itself. The result I'm seeking of would be something shown below :-

let people = [
    {
        "_id": 1,
        "name": "Person 1",
        "pets": [
            {
                "_id": 1,
                "name": "Tom",
                "category": "cat",
                "favFood": [
                    {
                        "_id": 1,
                        "name": "steamed fish"
                    },
                    {
                        "_id": 2, 
                        "name": "fried fish" // this is the new data inserted
                    }
                ]
            },
            {
                "_id": 2,
                "name": "Jerry",
                "category": "mouse",
                "favFood": [
                    {
                        "_id": 1,
                        "name": "cheese"
                    }
                ] 
            }
        ]
    }
]

is there any other way I can do this? Do I have to implement $elemMatch in this? If so, how can I do that here?

Answer 1

The query part matches documents, not elements, so { "_id": petID } is matching the person's _id. To match the pet's, use dotted notation like { "pets._id": petID }

Then use the positional operator $ to reference the first array element that matched the query part:

.updateOne(
    { "pets._id": petID },
    { $push: { "pets.$.favFood": newFavFood }}
)