Sorting By Field

Reduce Phase to Sort by Field

Contributed By: Francisco Treacy

Source File on GitHub

This a JavaScript MapReduce function that is actually written in CoffeeScript. Its original use was to sort by field on objects stored in Riak.

Here is what the functions looks like in CoffeeScript:

sort = (values, arg) ->
 field = arg?.by
 reverse = arg?.order is 'desc'
 values.sort (a, b) ->
   if reverse then [a,b] = [b,a]
   if a?[field] < b?[field] then -1
   else if a?[field] is b?[field] then 0
   else if a?[field] > b?[field] then 1

This compiles into JavaScript and looks like this:

var sort = function(values, arg) {
 var field = (typeof arg === "undefined" || arg === null) ? undefined : arg.by;
 var reverse = ((typeof arg === "undefined" || arg === null) ?
undefined : arg.order) === 'desc';
 values.sort(function(a, b) {
   if (reverse) {
     var _ref = [b, a];
     a = _ref[0];
     b = _ref[1];
   }
   if (((typeof a === "undefined" || a === null) ? undefined :
a[field]) < ((typeof b === "undefined" || b === null) ? undefined :
b[field])) {
     return -1;
   } else if (((typeof a === "undefined" || a === null) ? undefined :
a[field]) === ((typeof b === "undefined" || b === null) ? undefined :
b[field])) {
     return 0;
   } else if (((typeof a === "undefined" || a === null) ? undefined :
a[field]) > ((typeof b === "undefined" || b === null) ? undefined :
b[field])) {
     return 1;
   }
 });
};

Tips and tricks for using this functions

Here is how it's actually used with riak-js, a Node.js client.

db
 .add('airlines')
 .link({ bucket: 'flights', keep: false })
 .map('Riak.mapValuesJson')
 .reduce('Contrib.sort', { by: 'passengers', order: 'desc' })
 .run()

Default sorting is ascending (asc) so you can drop it. Specify desc for descending. The by property is required.