is it possible to use "$where" in mongodb aggregation functions

Question

I need to get the length of a string value in MongoDB using aggregation functions.

it works in

db.collection_name.find({"$where":"this.app_name.length===12"})

but when implanted to

db.collection_name.aggregate({$match: 
                                 {"$where":"this.app_name.length===12"}
                                 },
                             {
                             $group :
                                  {
                                  _id : 1,
                                   app_downloads : {$sum: "$app_downloads"}
                                  }
                             }

);

I got this result:

failed: exception: $where is not allowed inside of a $match aggregation expression

The question is: is it possible to use $where in aggregation functions? or is there any way of getting the length of a string value in aggregation function?

Thanks in advance Eric

possible duplicate of Select string length in mongodb

WiredPrairie
– WiredPrairie

2013-06-24 18:18:18 +00:00
Commented Jun 24, 2013 at 18:18 — WiredPrairie
– WiredPrairie, Commented Jun 24, 2013 at 18:18

Artem Mezhenin · Accepted Answer · 2013-06-24 16:25:31Z

5

MongoDB doesn't support $where in aggregation pipeline and hope this will never happen, because JavaScript slows things down. Never the less, you still have options:

1) Мaintain additional field(e.g. app_name_len) than will store app_name length and query it, when needed.

2) You can try extremely slow MapReduce framework, where you allowed to write aggregations with JavaScript.

answered Jun 24, 2013 at 16:25

Artem Mezhenin

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yAnTar · Accepted Answer · 2015-10-15 08:46:30Z

0

Today I had the same problem.

Mongodb doesn't support this.app_name.length, but you can do this condition with $regex - this is not very quick, but it still works.

{"app_name": { $regex: /^.{12}$/ }}

answered Oct 15, 2015 at 8:46

yAnTar

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ray · Accepted Answer · 2022-11-09 20:12:30Z

0

A simple way to achieve the behaviour expected of OP would be chaining up $expr with $strLenCP

db.collection.find({
  $expr: {
    $eq: [
      12,
      {
        $strLenCP: "$app_name"
      }
    ]
  }
})

Mongo Playground

answered Nov 9, 2022 at 20:12

ray

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