How to find longest and shortest length of a value for a field in mongoDb?

Question

The data type of the field is String. I would like to find the length of the longest and shortest value for a field in mongoDB.

I have totally 500000 documents in my collection.

Answer 1

In modern releases MongoDB has the $strLenBytes or $strLenCP aggregation operators than allow you to simply do:

Class.collection.aggregate([
  { "$group" => {
    "_id" => nil,
    "max" => { "$max" => { "$strLenCP" => "$a" } },
    "min" => { "$min" => { "$strLenCP" => "$a" } }
  }}
]) 

Where "a" is the string property in your document you want to get the min and max length from.

---

To output the minimum and maximum length, the best approach available is to use mapReduce with a few tricks to just keep the values.

First you define a mapper function which is just really going to output a single item from your collection to reduce the load:

map = Q%{
    function () {

      if ( this.a.length < store[0] )
        store[0] = this.a.length;

      if ( this.a.length > store[1] )
        store[1] = this.a.length;

      if ( count == 0 )
        emit( null, 0 );

      count++;

    }
}

Since this is working mostly with a globally scoped variable keeping the min and max lengths you just want to substitute this in a finalize function on the single document emitted. There is no reduce stage, but define a "blank" function for this even though it is not called:

reduce = Q%{ function() {} }

finalize = Q%{
    function(key,value) {
        return {
            min: store[0],
            max: store[1]
        };
    }
}

Then call the mapReduce operation:

Class.map_reduce(map,reduce).out(inline: 1).finalize(finalize).scope(store: [], count: 0)

So all the work is done on the server and not by iterating results sent to the client application. On a small set like this:

{ "_id" : ObjectId("543e8ee7ddd272814f919472"), "a" : "this" }
{ "_id" : ObjectId("543e8eedddd272814f919473"), "a" : "something" }
{ "_id" : ObjectId("543e8ef6ddd272814f919474"), "a" : "other" }

You get a result like this (shell output, but much the same for the driver ):

{
    "results" : [
            {
                    "_id" : null,
                    "value" : {
                            "min" : 4,
                            "max" : 9
                    }
            }
    ],
    "timeMillis" : 1,
    "counts" : {
            "input" : 3,
            "emit" : 1,
            "reduce" : 0,
            "output" : 1
    },
    "ok" : 1
}

So mapReduce allows the JavaScript processing on the server to do this fairly quickly, reducing your network traffic. There is no other native way at present for MongoDB to return a string length right now, so the JavaScript processing is necessary on the server.