db={users:[{email:"email.1@gmail.com"}],toBeUsers:[{email:"email.1@gmail.com",firstName:"firstName.1",lastName:"lastName.1",password:"Password"},{email:"email.2@gmail.com",firstName:"firstName.2",lastName:"lastName.2",password:"Password"},{email:"email.3@gmail.com",firstName:"firstName.3",lastName:"lastName.3",password:"Password"}]}

db.toBeUsers.aggregate([{"$merge":{"into":"users","on":"email","whenMatched":"merge","whenNotMatched":"insert"}}])