[mongodb mapreduce] string concatenate

http://kylebanker.com/blog/2009/12/mongodb-map-reduce-basics

 문자열 결합( string concatenate )

input data

db.test1.save({"keyword":"다음",   "date":"11/11"})

db.test1.save({"keyword":"네이버", "date":"04/21"})

db.test1.save({"keyword":"다음",   "date":"12/01"})

map function

m = function(){ emit(this.keyword, { merge_date:this.date}); }

reduce function

r = function(key, values){

   var all = [];

   values.forEach(function(x){

     all.push(x.merge_date)

   })

   return {"merge_date": all.join(", ")};

}

-> reduce 함수에 파라미터는 다음과 같이 넘어온다.

key : 다음,    values : [{ merge_date : "11/11"}, { merge_date : "12/01} ]

key : 네이버, values : [{ merge_date : "04/21"} ]

call map/reduce

db.test1.mapReduce(m,r, {out:"mr_result"}) 

output data

db.mr_result.find()

{ "_id" : "네이버", "value" : { "merge_date" : "04/21" } }

{ "_id" : "다음", "value" : { "merge_date" : "11/11, 12/01" } }