return db.collection('people')

.then( (collection) => {
    // Store reference to collection for future use
    peopleCollection = collection;

    return collection.find({a:1})
})

.then( (people) => {

    // Process each people
    return people.each( (person) => {

        person.b = 2;

        // Where peopleCollection is a reference to my collection
        return peopleCollection.update({_id: person._id}, person)
    })
})