mongodb聚合函数(二)
t( { _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 : [] } );
> // map function
> m = function(){
... this.tags.forEach(
... function(z){
... emit( z , { count : 1 } );
... }
... );
...};
> // reduce function
> r = function( key , values ){
... var total = 0;
... for ( var i=0; i
... total += values[i].count;
... return { count : total };
...};
> res = db.things.mapReduce(m,r);
> res
{"timeMillis.emit" : 9 , "result" : "mr.things.1254430454.3" ,
"numObjects" : 4 , "timeMillis" : 9 , "errmsg" : "" , "ok" : 0}
> db[res.result].find()
{"_id" : "cat" , "value" : {"count" : 3}}
{"_id" : "dog" , "value" : {"count" : 2}}
{"_id" : "mouse" , "value" : {"count" : 1}}
> db[res.result].drop()