Map reduce on mongodb returns recursive result

I have a collection of real estates and I run map reduce on mongodb to calculate some basic statistic. Everything works fine locally, but when I run task on mongohq I get back recursive result.

Let me simplify things a bit, instead of real reduce fn, say reduce is:

function(key, values) {
  return { values: values };
}

Map function:

function() {
  emit('price', this.price);
}

When I run task locally, output looks like:

{
  "values": [
    1024.1712707182319,
    661.0377201728149,
    651.5957446808511,
    1553.7073816617014,
    1128.664171911323
  ]
}

Now fun part, when I run it on production db, output looks like:

{
  "values": [
    {
      "values": [
        {
          "values": [
            1561.5615615615618,
            1026.2054507337525,
            1428.5714285714287
          ]
        },
        1092.1177587844254,
        1040.2010050251256,
        1547.6190476190477
      ]
    }
  ]
}

Any idea what could be wrong here?

The culprit is probably your reduce function.

https://docs.mongodb.org/manual/reference/command/mapReduce/#dbcmd.mapReduce

the reduce function must be idempotent. Ensure that the following statement is true:

reduce( key, [ reduce(key, valuesArray) ] ) == reduce( key, valuesArray )

the reduce function should be commutative: that is, the order of the elements in the valuesArray should not affect the output of the reduce function, so that the following statement is true:

reduce( key, [ A, B ] ) == reduce( key, [ B, A ] )

Hope this helps,