Adds a sum($selector, $field) method to the $pages class. Use for very fast summations across large data sets.
This is a (very) slightly rewritten version of ESRCH's original PagesSum module for Processwire CMS/CMF. You can find the orignal article that introduced it here. The only change introduced is that I've used a closure for the hook function.
This is ESRCH's work. I've just put it on github to make it a little more accessible (it's buried deep in the Processwire forums.)
The module uses ProcessWire's ability to extend its classes by adding new hook functions to them. It adds one method sum to the Pages class. Thus, anywhere you get access to the $pages class, you can get a sum worked out from any subset of pages. The subset is defined using PWs selectors.
The summation is fast as it uses MySQL to sum the underlying data rather than having to iterate over every page in the selected set and pull out a value.
In the Processwire modules page, click the "New" tab and either enter PagesSum into the box or choose the option to install from a zip file. The zip file to use is the master branch zip from the gihub repository, here.
If you wanted to sum a field called 'costs' across all pages using the 'job' template, you'd simply do this...
<p>Job costs £<?php echo $pages->sum('template=jobs', 'costs'); ?></p>
And if you wanted things a little more formatted, just run that through number_format first...
<p>Job costs £<?php echo number_format($pages->sum('template=jobs', 'costs'), 2); ?></p>