map reduce example 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 : [] } ); var map = function () { var things = this ; things.tags.forEach( function (elem) { emit( elem , { count : 1 } ); } ); }; var reduce = function ( key , values ) { var total = 0; for ( var i=0; i<values.length; i++ ) { total += values[i].count; } return { count : total }; }; var res = db.things.mapReduce(map, reduce, { out : "myoutput" } ); insert example db.things.insert( { a : 'asdf\n asdf' } ); master (f7df4c7) $ mongod --v 2014-07-31T10:56:53.753+0100 I COMMANDS [conn1] command test.$cmd command: mapReduce { mapreduce: "things", map: function () { var things = this; things.tags.forEach( function(ele..., reduce: function ( key , values ) { var total = 0; for ( var i=0; i<values..., out: "myoutput" } planSummary: COLLSCAN keyUpdates:0 numYields:0 locks(micros) r:2756 w:1818 reslen:205 24ms 2014-07-31T10:57:30.833+0100 I COMMANDS [conn1] insert test.things query: { _id: ObjectId('53da130a998eaf05e7f22c7c'), a: "asdf asdf" } ninserted:1 keyUpdates:0 numYields:0 locks(micros) w:81 0ms 2.6.3 $ mongod --v 2014-07-31T10:39:52.109+0100 [conn1] command test.$cmd command: mapReduce { mapreduce: "things", map: function () { var things = this; things.tags.forEach( function(ele..., reduce: function ( key , values ) { var total = 0; for ( var i=0; i<values..., out: "myoutput" } planSummary: COLLSCAN keyUpdates:0 numYields:0 locks(micros) r:6729 w:5992 reslen:175 71ms 2014-07-31T10:40:00.955+0100 [conn1] insert test.things query: { _id: ObjectId('53da0ef0998eaf05e7f22c7a'), a: "asdf asdf" } ninserted:1 keyUpdates:0 numYields:0 locks(micros) w:83 0ms