[MongoDB] 顯示collection 內的column

利用MongoDB 在編程上帶來不少方便, 但稍一不慎就會令到結構變得難以複雜. 示範中會利用mongoDB 讀取collection 中的property name.

1. 加入Function isArray() 及m_sub() 作檢查Array 及recursive load.

   isArray = function (v) {
     return v && typeof v === 'object' && typeof v.length === 'number' && !(v.propertyIsEnumerable('length'));
   }
   
   m_sub = function(base, value){
     for(var key in value) {
       if(key != "_id") { 
           emit(base + "." + key, null);
       if( isArray(value[key]) || typeof value[key] == 'object'){
         m_sub(base + "." + key, value[key]);
       }
   }
     }
   }
   
   db.system.js.save( { _id : "isArray", value : isArray } );
   db.system.js.save( { _id : "m_sub", value : m_sub } );
   

2.  設定map reduce 以決定存取資料及filter 設定.

   map = function(){
     for(var key in this) {
       emit(key, null);
       if( isArray(this[key]) || typeof this[key] == 'object'){
         m_sub(key, this[key]);
       }
     }
   }
   
   reduce = function(key, stuff){ return null; }

3. 存取collection.

   var collectionName="<<Collection Name>>";
   var mr = db.runCommand({
       "mapreduce" : collectionName, 
       "map" : map, 
       "reduce" : reduce,
       "out": "things" + "_keys"});
   var result=db[mr.result].distinct("_id");
   print(result);