Schema

// grab the things we need
var mongoose = require('mongoose');
var Schema = mongoose.Schema;


var postSchema = new Schema({
	title  : {type:String, required:true},
	content : {type:String, required:true},
	categories:{
		name: {type:String, required:true},
	}

});



var Post = mongoose.model('Post', postSchema);

// make this available to our users in our Node applications
module.exports = Post;

var postSchema = new Schema({
    title  : {type:String, required:true},
    content : {type:String, required:true},
    categories: [
        { name : {type:String, required:true}},
    ]

});

ahh eu sei vai ficar duplicado, no modelo relacional eu faria um join e blz, vale salientar, que mongoDB visa performance de busca, db relacional quer economizar espaço em disco, que não faz muito sentido em um blog por exemplo.

var array = ['node','java','angular','node','jenkis'];

var arrayfilter = array.filter(function (item, i, ar) { return ar.indexOf(item) === i;})
["node", "java", "angular", "jenkis"]