query3.js

//query3
//create a collection "cities" to store every user that lives in every city
//Each document(city) has following schema:
/*

{
  _id: city
  users:[userids]
}


*/

function cities_table(dbname) {
    db = db.getSiblingDB(dbname)

    db.users.aggregate([
    	{$group: {_id: "$hometown.city", users: {$push: "$user_id"}}},
    	{$out: "cities"}
    ]);
}