Mongodb aggregate cannot find operator

  mongodb, question

I don’t understand at all.

db.users.aggregate(
 [
 {
 $group : {
 _id: { $sum: 1 }
 }
 }
 ]
 )
Error: Printing Stack Trace
 at printStackTrace (src/mongo/shell/utils.js:37:15)
 at DBCollection.aggregate (src/mongo/shell/collection.js:897:9)
 at (shell):1:10
 Fri Sep 19 18:51:23.623 aggregate failed: {
 "errmsg" : "exception: invalid operator '$sum'",
 "code" : 15999,
 "ok" : 0

_idIt is used for grouping. If you want to use $sum, you need to change the field name.

http://docs.mongodb.org/manual/reference/operator/aggregation/sum/#grp._S_sum