db.pets.updateOne(
    {_id: "1"},
    {
        $set: {chunky: true}
    }
)


db.pets.find(
    {neutuered: {$exists: false}
})

// still only updates one
db.pets.update(
    {neutuered: {$exists: false}},
    {
        $set: {
            neutuered: false,
        }
    }
)

// updates many
db.pets.updateMany(
    {neutuered: {$exists: false}},
    {
        $set: {
            neutuered: false,
        }
    }
)