Find count of maximum consecutive records based on one field in Mongodb Query

mapreduce, mongodb, mongoid

Solution

You can do this mapReduce operation.

First the mapper:

var mapper = function () {


    if ( this.flag == true ) {
        totalCount++;
    } else {
        totalCount = 0;
    }

    if ( totalCount != 0 ) {
        emit (
        counter,
        {  _id: this._id, totalCount: totalCount }
    );
    } else {
      counter++;
    }

};

Which keeps a running count of the total times that the `true` value is seen in flag. If that count is more than 1 then we emit the the value, also containing the document `_id`. Another counter which is used for the key is incremented when the flag is `false`, in order to have a grouping "key" for the matches.

Then the reducer:

var reducer = function ( key, values ) {

    var result = { docs: [] };

    values.forEach(function(value) {
        result.docs.push(value._id);
        result.totalCount = value.totalCount;
    });

    return result;

};

Simply pushes the `_id` values onto a result array along with the totalCount.

Then run:

db.people.mapReduce(
    mapper,
    reducer,
   { 
       "out": { "inline": 1 }, 
       "scope": { 
           "totalCount": 0, 
           "counter": 0 
       }, 
       "sort": { "updated_at": 1 } 
   }
)

So with the `mapper` and `reducer` functions, we then define the global variables used in "scope" and pass in the "sort" that was required on `updated_at` dates. Which gives the result:

{
    "results" : [
        {
            "_id" : 1,
            "value" : {
                "docs" : [
                     3,
                     4
                 ],
                 "totalCount" : 2
            }
        },
        {
            "_id" : 2,
            "value" : {
            "docs" : [
                7,
                8,
                5
             ],
             "totalCount" : 3
             }
        }
    ],
    "timeMillis" : 2,
    "counts" : {
            "input" : 7,
            "emit" : 5,
            "reduce" : 2,
            "output" : 2
    },
    "ok" : 1,
}

Of course you could just skip the `totalCount` variable and just use the array length, which would be the same. But since you want to use that counter anyway it's just added in. But that's the principle.

So yes, this was a problem suited to mapReduce, and now you have an example.

Problem

Original source