function my_aggregate() {
db.zips.aggregate([{ $match: { state: { $in: [ "CA", "NY" ] } }},{ $group:{ _id : {  "state" : "$state","city" : "$city" }, pop : { $sum: "$pop"}}},{ $match: {  pop: { $gt: 25000 } }},{$group:{ _id : {   "city" : "$city"}, pop : { $avg: "$pop"}}}]);


}