MongoDB Map/Reduce for Chariot Day

nosql-top-dogs.js

db.nosqlrep.insert( { _id : 1, votes : ['riak', 'couchdb', 'cassandra'] } );
db.nosqlrep.insert( { _id : 2, votes : ['redis', 'onetick', 'couchdb'] } );
db.nosqlrep.insert( { _id : 3, votes : ['redis', 'riak', 'couchdb'] } );
db.nosqlrep.insert( { _id : 4, votes : ['voltdb', 'mongodb', 'hazelcast']  } );
db.nosqlrep.insert( { _id : 5, votes : ['hazelcast', 'riak', 'redis']  } );
db.nosqlrep.insert( { _id : 6, votes : ['redis', 'cassandra', 'onetick']  } );
db.nosqlrep.insert( { _id : 7, votes : []  } );

print( "\ncommunity votes" )
print( "---------------" )
db.nosqlrep.find().forEach( printjson )

map = function() {
    this.votes.forEach(
      function( z ) {
        emit( z, { vote : 1 } );
      }
    );
};

reduce = function( key, values ) {
    var total = 0;
    for ( var i = 0; i < values.length; i++ )
        total += values[i].vote;
    return { votes : total };
};

stats = db.nosqlrep.mapReduce( map, reduce, { out : "topdogs" } );

print( "\nhere are the stats of the map/reduce:" )
print( "-------------------------------------" )
printjson( stats );

print( "\nand the top dogs are..." )
print( "-----------------------" )
print( "// in here \t\t=> db.topdogs.find().sort( { value: -1 } )" )
print( "// single DB rank \t=> db.nosqlrep.distinct( \"_id\", { votes: \"riak\" } ).length\n" )

run-nosql-top-dogs.sh

$ mongo --shell top-nosql-dogs.js 
MongoDB shell version: 2.0.2
connecting to: test
type "help" for help

community votes
---------------
{ "_id" : 1, "votes" : [ "riak", "couchdb", "cassandra" ] }
{ "_id" : 2, "votes" : [ "redis", "onetick", "couchdb" ] }
{ "_id" : 3, "votes" : [ "redis", "riak", "couchdb" ] }
{ "_id" : 4, "votes" : [ "voltdb", "mongodb", "hazelcast" ] }
{ "_id" : 5, "votes" : [ "hazelcast", "riak", "redis" ] }
{ "_id" : 6, "votes" : [ "redis", "cassandra", "onetick" ] }
{ "_id" : 7, "votes" : [ ] }

here are the stats of the map/reduce:
-------------------------------------
{
	"result" : "topdogs",
	"timeMillis" : 17,
	"counts" : {
		"input" : 7,
		"emit" : 18,
		"reduce" : 6,
		"output" : 8
	},
	"ok" : 1,
}

and the top dogs are...
-----------------------
// in here 		=> db.topdogs.find().sort( { value: -1 } )
// single DB rank 	=> db.nosqlrep.distinct( "_id", { votes: "riak" } ).length

> db.topdogs.find().sort( { value: -1 } )
{ "_id" : "redis", "value" : { "votes" : 4 } }
{ "_id" : "couchdb", "value" : { "votes" : 3 } }
{ "_id" : "riak", "value" : { "votes" : 3 } }
{ "_id" : "cassandra", "value" : { "votes" : 2 } }
{ "_id" : "hazelcast", "value" : { "votes" : 2 } }
{ "_id" : "onetick", "value" : { "votes" : 2 } }
{ "_id" : "mongodb", "value" : { "vote" : 1 } }
{ "_id" : "voltdb", "value" : { "vote" : 1 } }
>