Map-Reduce for finding Co-authors

db.test.drop();                                                                 
db.test.save({authors: ["mike"]});                                              
db.test.save({authors: ["mike", "eliot"]});                                     
db.test.save({authors: ["eliot", "dwight", "steve", "mike"]});                  
db.test.save({authors: ["steve", "mike"]});                                     
                                                                                
var res = db.test.mapReduce(                                                    
              function() {                                                      
                  var that=this;                                                
                  that.authors.forEach(function(a){                             
                      that.authors.forEach(function(b){                         
                          if(a !== b){                                          
                              var doc = {};                                     
                              doc[a] = b;                                       
                              emit(doc, 1);                                     
                          };                                                    
                      });                                                       
                  });                                                           
              },                                                                
              function(k, vals) {                                               
                  var sum=0;                                                    
                  for(var i in vals) sum += vals[i];                            
                  return sum;                                                   
              });                                                               
                                                                                
db[res.result].find().forEach(                                                  
    function(doc) {                                                             
        printjson(doc);                                                         
    });