在MongoDB中创建MapReduce查询

Question

I have a slight problem. Until now, I'm not so deep in the Mongo DB stuff. More like the SQL guy, but now I have to crate a Map Reduce query.

My data looks like this:

Every student has multiple lectures. Now I want a list of all lectures (no duplicates) and the number of students visiting it.

function() {
emit(this.vorlesungen, this._id);
};


function(VL, students) {
                      return Array.sum(students);
                  };

This gives me some strange result I can´t figure out.

Some hints would be great.

Answer 1

I'm not sure if this is the data that you're expecting but give it a try:

db.collection.aggregate([
   {$unwind : "$vorlesungen"},
   {$group : {_id : "$vorlesungen", studentCount: {$sum : 1}}}
])

Answer 2

I found a aulution by myself:

Map:

 function() { this.vorlesungen.forEach(function (elem) { for(vlname in elem) { emit(vlname, 1); } }); 

Reduce:

 function(key, values) { return Array.sum(values); }