gistfile1.js

function returnPosts(postKeys, req, res){
	
  //init empty inputs array
  var inputs = [];
    //postKeys is a list of riak keys. loop over them
    for (var i=0;i < postKeys.length;i++){
      //bk (bucket/key) is created as it's own array of two
      //strings ["bucket","key"] (i use a "posts" bucket in my app)
      var bk = ["posts", postKeys[i].toString() ];
      //bk array is pushed onto the inputs array
      inputs.push(bk);
    }

  //construct map function 
  var map = function(v, keydata, args) {
    //v is the full value of data kept in riak
    //your data plus meta data
    //check riak wiki m/r page for an example of what 'v' looks like
    if (v.values) {
      //init an empty return array
      var ret = [];
      //set 'o' (for object) equal to the data portion
      //Riak.mapValuesJson is an internal riak js func
      o = Riak.mapValuesJson(v)[0]; 
      //interesting part for the sorting. 
      //pull the last modified datestamp string out of the meta data
      //and turn it into an int	    
      o.lastModifiedParsed = Date.parse(v["values"][0]["metadata"]["X-Riak-Last-Modified"]);
      //i also return the key just for good measure which i use elsewhere in my app
      o.key = v["key"];
      //push the 'o' object into the ret array
      ret.push(o);
      return ret;
    } else { //if no value return an empty array
      return [];
    }
  };

  //construct reduce function	
  var reduceDescending = function ( v , args ) {
    //by default sort() sorts elements ascending, alpha.
    //we want numeric sort so we provide a numeric sort function
    //there is a riak builtin func but it expects an array of numeric values, 
    //not the numeric nested in an object 
    //here I return in DESC order, if you want ASC order rewrite return to 'a-b' 
    v.sort ( function(a,b) { return b['lastModifiedParsed'] - a['lastModifiedParsed'] } );
    return v
  };

  //riak is my connection to riak via nodejs	
  riak
    //call map phase passing in map function
    .map(map)
    //call reduce phase passing in reduce function
    .reduce(reduceDescending)
    //execute the m/r with the array of keys created earlier
    //you could simply m/r over an entire bucket by replacing 'inputs' with "bucket_name"
    //ymmv depending on total keys in your system, not recommended  
    .run(inputs)(function(response) {
      res.simpleJSON(200, response );
    });
	
}