Group and count in MongoDB - mongodb

Question

I'm trying to group and count the amount of elements for each group in mongodb 2.0.1, but with no success so far.
My DB schema looks like :
{
"_id" : ObjectId("4ece7544853b4b0941000000"),
"ResultSet" : {
"Results" : [
{
"quality" : 87,
"state" : "Franche-Comté"
}
]
}
}
I've been trying all sort of methods, following different tutorials, but it is each time the same result : an only null group... which I don't understand why.
The best query I have written so far is the following :
db.extract_2000.group( {
cond: { "ResultSet.Results.quality": {$exists: true} },
key: {"ResultSet.Results.state": true},
reduce: function(obj, glob) { glob.total++; glob.quality += obj.ResultSet.Results.quality },
initial: { total: 0, quality: 0 },
finalize: function(glob) {glob.avgquality = glob.quality / glob.total}
})
Which returns (once again) :
[
{
"ResultSet.Results.state" : null,
"total" : 2000,
"quality" : NaN,
"avgquality" : NaN
}
]
What am I doing wrong ?

Answer 1

This simply won't work as written. The key problem is here: key: {"ResultSet.Results.state": true}. ResultSet.Results is an array. When you ask for ResultSet.Results.state you are implying some type of for loop be done here. The group command is simply not capable of this.
Instead try the following M/R:
map = function() {
// Note that we emit once per result
foreach(var i in ResultSet.Results) {
key = this.ResultSet.Results[i];
value = { count: 1,
quality: this.ResultSet.Results[i].quality,
avg_quality: 0
};
emit(key, value);
}
}
reduce = function(key, values) {
// note that results has same fields as emitted value
var results = { count: 0, quality: 0, avg_quality: 0 };
foreach(var i in values){
results.count += values[i].count;
results.quality += values[i].quality;
// ignore avg_quality, we don't use it
}
return results;
}
You will also have to write a finalize for the average.
finalize = function(key, value) {
if (value.count > 0)
value.avg_quality = value.quality / value.count;
return value;
}

Answer 2

The map function
map = function() {
for(var i in this.Results) {
emit(this.Results[i].state,
{quality: this.Results[i].quality, total: 1, avgquality: 0}
);
}
}
The reduce function
reduce = function(key, values) {
var data = {quality: 0, total: 0, avgquality: 0};
for(var i=0; i<values.length; i++) {
data.quality += values[i].quality;
data.total += values[i].total;
}
return data;
}
In finalize function only calculate the avg.