MongoDB CRUD and MapReduce

example.js

db.foo.insert( { name: "Mario", fach: ["E", "D"] } );
db.foo.insert( { name: "Philipp", fach: ["PR", "D"] } );

db.foo.find();
db.foo.find( { name: "Mario" } );
db.foo.find( { fach: { $in : [ "E", "M" ] } } );

db.foo.update( { name: "Mario" }, { $push: { fach: "M" } } );
db.foo.find( { name: "Mario" } );

db.foo.ensureIndex( { name : 1 } );

db.foo.remove( { "name" : "Philipp" } );
db.foo.find();


db.things.insert( { _id : 1, tags : ['dog', 'cat'] } );
db.things.insert( { _id : 2, tags : ['cat'] } );
db.things.insert( { _id : 3, tags : ['mouse', 'cat', 'dog'] } );
db.things.insert( { _id : 4, tags : []  } );
db.things.insert( { _id : 5 } );

function map() {
    if( typeof this.tags != 'undefined' ) {
      this.tags.forEach( function( tag ){
              emit( tag , { count : 1 } );
      });
    }
}

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

res = db.things.mapReduce(map, reduce, { out : "myoutput" } );

print("Try db.myoutput.find().sort( { \"value.count\" : -1 } );");