Question

Just started mongo and started having issue with querying already. i have a collection called ‘externalTransaction’ and i want to write a equivalent of this mysql query:

select transactionCode, 
       sum(amount) as totalSum, 
       count(amount) as totalCount 
from externalTransaction 
where transactioncode in ('aa','bb','cc') 
group by sum(amount)

below is my attempt:

{
 "collectionName": "externalTransaction",
  sort: {transactionCode:-1},
  query: {this._id: {$in:['aa','bb','cc']}},
  mapReduce:{
   'map': 'function(){

        emit(this.transactionCode, this.amount);

   }',
 'reduce': 'function(key, values){
    var result = {count: 0, sum: 0.0};

    values.forEach(function(value) {
      result.count++
       result.sum += value.amount;
    });
    return result;
 }',
   'out' : 'sumAmount'
 }

}

the above query give me a result set looking like this:

_id     value.count        value.sum
ct      2.0                NaN
bb      40.0           NaN
fg      71.0           NaN
fd      36.0           NaN
sd      5.0                NaN
as      4.0            NaN
aa      71.0           NaN
df      4.0                NaN
cc      10.0               NaN

From the documentation with the version 2.0.6 i can’t use the aggregation framework just yet so how to handle simple queries like mine in mongo. thanks for reading and excuse the triviality of my question.

Answer 1

You have a few errors in your map and reduce functions. First, in map you emit a simple number, and in reduce you try to take amount of a number. I bet, it doesn’t have that property. Second, outputs of map and reduce must be uniform, because reduce is supposed to be runnable over partially reduced results. Try these functions:

var map = function() {
  emit(this.transactionCode, {sum: this.amount, count: 1})
}

var reduce = function(k, vals) {
  var result = {sum: 0, count: 0};

  vals.forEach(function(v) {
    result.sum += v.sum;
    result.count += v.count;
  });
  return result;
}