使用聚合时如何在运行限制之前获取记录总数

Question

How do I get the total number of records before I run limit when using Aggregation?

stage 1

{
  $text: { 
    $search: 'blah', 
    $caseSensitive: false 
}

What can i do here to get the total number of records, so I can return as part response.

stage 3

{ 
  $limit : 50 
}

Answer 1

This would be a hack. As far as i understand your requirement, you need to get the count of total matching records before limiting them.

You can do this with below aggregation pipeline.

db.session.aggregate([{'$match':{'some-field':'some-value'}}, {'$group':{'_id':null, 'records':{'$push':'$$ROOT'}}}, {'$project':{'records':1, 'total_records':{'$size':'$records'}}}, {'$unwind':'$records'}, {'$limit':1}],{allowDiskUse:true})

total_records field will be the total matching documents count. And Don't forgot to pass allowDiskUse parameter. You might also need to use intermittent projection stage to alter documents as per your need.