aggregate with skip can't work

Hi, guys, please help me out, thank you very much!

Schema

ReportSchema = new Schema
  user:type:Schema.ObjectId,ref:'User',index:sparse:true
  report:[
    qid:type:Schema.ObjectId,ref:'Question'
    value:Schema.Types.Mixed
    remark:String
  ]
  created_at:Date

Aggregate when skip=0 works fine, but when skip!=0, I got nothing. I’m sure there’re pages data.

Report.aggregate()
    .unwind('report')
    .match('report.qid':new ObjectId(data.qid))
    .project(
      value:'$report.value'
      remark:'$report.remark'
      user:'$user'
    )
    .sort('created_at':-1)
    .limit(limit)
    .skip(skip)
    .exec (err, result)->

[mani95lisa]

I resolved this to make skip before limit, is it a bug?

.skip(skip)
.limit(limit)

[vkarpov15]

@mypawdesign is right. In aggregate, $limit limits the number of documents sent to the next aggregation state, and $skip skips the first N documents, so if $skip is after $limit and $skip >= $limit, you won’t get any results. In short, this is expected behavior in MongoDB.