MapReduce, MongoDB and node-mongodb-native - mongodb

Question

I'm using the node-mongodb-native library to run a MapReduce on MongoDB (from node.js).
Here's my code:
var map = function() {
emit(this._id, {'count': this.count});
};
var reduce = function(key, values) {
return {'testing':1};
};
collection.mapReduce(
map,
reduce,
{
query:{ '_id': /s.*/g },
sort: {'count': -1},
limit: 10,
jsMode: true,
verbose: false,
out: { inline: 1 }
},
function(err, results) {
logger.log(results);
}
);
Two questions:
1) Basically, my reduce function is ignored. No matter what I put in it, the output remains just the result of my map function (no 'testing', in this case). Any ideas?
2) I get an error unless an index is defined on the field used for the sort (in this case - the count field). I understand this is to be expected. It seems inefficient as surely the right index would be (_id, count) and not (count), as in theory the _id should be used first (for the query), and only then the sorting should be applied to the applicable results. Am I missing something here? Is MongoDB inefficient? Is this a bug?
Thanks! :)

Answer 1

The reason why the reduce function is never called is due to you emitting a single value for each key so there is no reason for the reduce function to actually execute. Here is an example of how you trigger the reduce function
collection.insert([{group: 1, price:41}, {group: 1, price:22}, {group: 2, price:12}], {w:1}, function(err, r) {
// String functions
var map = function() {
emit(this.group, this.price);
};
var reduce = function(key, values) {
return Array.sum(values);
};
collection.mapReduce(
map,
reduce,
{
query:{},
// sort: {'count': -1},
// limit: 10,
// jsMode: true,
// verbose: false,
out: { inline: 1 }
},
function(err, results) {
console.log("----------- 0")
console.dir(err)
console.dir(results)
// logger.log(results);
}
);
Notice that we are emitting by the "group" key meaning there is n >= 0 entries grouped by the "group" key. Since you are emitting _id each key is unique and thus the reduce function is not needed.
http://docs.mongodb.org/manual/reference/command/mapReduce/#requirements-for-the-reduce-function