Mongo update across databases

updateAcrossDatabases.js

// Add employee.badge_number to matching person in another database collection
db.getSiblingDB('databaseA').getCollection('employees').find({}).forEach(function(employee1) {
    db.getSiblingDB('databaseB').getCollection('people').update(
        {"_id": employee1._id},
        {
            "$set": {
                "badge_number": employee1.badge_number
              }
        }
    )
});