mongoDB中nested sql selects的替代

// I want to make an efficient query to find all users who have theiruserids listed in a usergroup.// Ideally I want to make this as a single request to mongodb.// What I want corresponds to nested selects in SQL.// I have tried this in the mongo shell:db.user.save({_id:"u1", Name:"u1 name"});db.user.save({_id:"u2", Name:"u1 name"});db.user.save({_id:"u3", Name:"u3 name"});db.usergroup.save({_id:"g1", Users: ["u2","u3"]});// This is the select I want to do, but without hardcoding the["u2","u3"] array:db.user.find({_id:{$in:["u2","u3"]}}).forEach(printjson);// So the question is how to get the array of userids in the $inoperator extracted with a query such that the enite query can be madewith a single request.// A "nested query" like this does not work://db.user.find({_id:{$in:db.usergroup.find({_id:"g1"},{_id:0,Users:1})}}).forEach(printjson);//Gives this error://Tue Mar 27 06:17:41 uncaught exception: error: { "$err" : "invalidquery", "code" : 12580 }//failed to load: mongoNestedSelect.js

解決方案：

db.usergroup.find({"_id":"g1"}).forEach(function(doc){  db.user.find({_id:{$in:doc.Users}}).forEach(printjson);})