Reduce Phase to Sort 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, Riak’s 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.
