[mongodb] group by

* 몽고DB에서 group by (단순)

sql	몽고DB
select     basetime, count(basetime) as csum from     fileinfos group by      basetime	db.fileinfos.group(  { key : {basetime:true},   reduce: function(obj, prev) { prev.csum += 1; },   initial: { csum: 0 }  });

* 몽고DB에서 group by (그룹대상 필드 가공하여 group by)

sql	몽고DB
select     basetime, count(basetime) as csum from     fileinfos group by      substr(basetime,1, 6)	db.fileinfos.group(  { keyf: function(obj) {return { basetime : obj.basetime.substr(0,6) };},   reduce: function(obj, prev) { prev.csum += 1; },   initial: { csum: 0 }  });

* mongoid를 이용해서 (ROR)에서 할때 참고

 @grouptest = Fileinfo.collection.group(  
  { :keyf=> "function(obj) {return { basetime : obj.basetime.substr(0,6) };}",
   :reduce=> "function(obj, prev) { prev.csum += 1; }",
   :initial=> { csum: 0 }
  }
 )

※ " 와 :  를 주의하여 작성할것