完全不明白
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
_id
是用来 GROUP(分组) 的,如果你要用 $sum, 需要换一个字段名。
http://docs.mongodb.org/manual/reference/operator/aggregation/sum/#grp._S_sum