Join using two map reduces for stackoverflow ( https://github.com/dsevilla/bdge/blob/master/mongo/sesion4.ipynb )

join_using_map_reduce.js

db.comments.mapReduce(
	function(){
		emit(this.PostId, { Comments: [ this ]})
	},
	function(key, val){
		var first = val.shift();
		var comments = val.reduce(function(valorAnterior, valorActual){
			return Array.prototype.concat.apply( valorAnterior, valorActual.Comments);
		}, first.Comments );
		return {
			Comments: comments
		};
	},
	{
		out: "post_comments"
	}
);

db.posts.mapReduce(
	function(){
		emit(this.Id, this );
	},
	function(key, val){
		return val.reduce(function(valorAnterior, valorActual){
			for (var attr in valorActual){
				if (typeof valorAnterior[attr] === 'undefined'){
					valorAnterior[attr] = valorActual[attr];
				} else if (Array.isArray(valorAnterior[attr])){
					valorAnterior[attr].push( valorActual[attr] );
				} else {
					valorAnterior[attr] = [ valorAnterior[attr], valorActual[ attr] ];
				}
			}
			return valorAnterior;
		}, {});
	},
	{
		out: {reduce: 'post_comments'}
	}
);