MongoDB/Mongoose: Can't put simplest MapReduce to work - mongodb

Question

Hello all I'm trying to do is to get the count of each distinct departmentType:
fnMap = function() {
emit(this.departments.departmentType, {typeCount:1} );
}
fnReduce = function(key, values) {
var result = {typeCount: 0};
values.forEach(function(value) {
result.typeCount += value.brandCount;
});
return result;
};
var command = {
mapreduce : "clients",
query : {"departments.departmentType": {$exists: true}},
map : fnMap.toString(),
reduce : fnReduce.toString(),
//sort: {"departments.departmentType":1},
out: {inline: 1}
};
mongoose.connection.db.executeDbCommand(command, function(err, dbres) {
});
When executing the command, dbres.documents[0].results only contains 1 item with the total number of departmentTypes, instead of several items one for each departmentType with its count.
Any ideas what am I doing wrong?
Also, when I uncomment the SORT line, I get error "db assertion failure: could not create cursor over...", I believe the field name is written correctly.

Answer 1

Mongoose v3 has now a Model.mapreduce() function (see doc).
The full example shown is:
var o = {};
o.map = function () { emit(this.name, 1) }
o.reduce = function (k, vals) { return vals.length }
o.out = { replace: 'createdCollectionNameForResults' }
o.verbose = true;
User.mapReduce(o, function (err, model, stats) {
console.log('map reduce took %d ms', stats.processtime)
model.find().where('value').gt(10).exec(function (err, docs) {
console.log(docs);
});
})
The problem with count i believe is because in your fnReduce() function you are summit the results instead of displaying them in an array.

Answer 2

You can use:
db.clients.distinct("departments.departmentType")
That will give an array with all the distinct departmentType values.
There were two problems in your map/reduce. One is brandCount in reduce rather than typeCount. But more importantly, you are trying to emit once per document, when you need to emit once per department array element. Corrected (and slightly simplified) code:
> fnMap = function () {
this.departments.forEach(
function (d) {
emit(d.departmentType, 1);
}
);
}
> fnReduce = function (key, values) {
var result = 0;
values.forEach(
function (value) {result += value;});
return result;
}