MongoDB shell

gistfile1.sh

> db.things.insert( { _id : 1, tags : ['tag1', 'tag2'] } );
> db.things.insert( { _id : 2, tags : ['tag2'] } );
> db.things.insert( { _id : 3, tags : ['tag1', 'tag2', 'tag3'] } );
> db.things.insert( { _id : 4, tags : []  } );

> // función de mapeo
> m = function(){
...    this.tags.forEach(
...        function(z){
...            emit( z , { count : 1 } );
...        }
...    );
...};

> // función reductora
> r = function( key , values ){
...    var total = 0;
...    for ( var i=0; i<values.length; i++ )
...        total += values[i].count;
...    return { count : total };
...};

> res = db.things.mapReduce(m, r, { out : "myoutput" } );
> res
{
   "result" : "myoutput",
   "timeMillis" : 12,
   "counts" : {
      "input" : 4,
      "emit" : 6,
      "output" : 3
   },
   "ok" : 1,
}
> db.myoutput.find()
{"_id" : "tag1" , "value" : {"count" : 3}}
{"_id" : "tag2" , "value" : {"count" : 2}}
{"_id" : "tag3" , "value" : {"count" : 1}}

> db.myoutput.drop()