Sort Tags by popularity, or: how to sort a DataObject by count of many_many relation

Today I did some minor updates on a site with a blog and wanted to show the most popular tags. But how can I sort a DataObject by the number of it's $many_many or $belongs_many_many relations?

Stuck on this issue I asked on IRC and Barry told pointed me to the right query, how it would work with good ol' plain SQL:

SELECT bt.ID, bt.Title, COUNT( * )
FROM BlogTag bt
LEFT JOIN BlogPost_Tags bpt ON bpt.BlogTagID = bt.ID

OK, there we go. Creating a DataList and a left join is pretty straigtforward (once you know your table names), so i got this pretty quickly:

$tags = BlogTag::get()
	->leftJoin('BlogPost_Tags','bpt.BlogTagID = BlogTag.ID','bpt')
	->sort('Count(*) DESC')

which is quite useless now, as we have to GROUP BY ID. But unfortunaltey DataList doesn't have a simple group by method.

Am I stuck? Can't I do this (maybe not so) simple query with my beloved SilverStripe ORM?

Well, each DataList produces a DataQuery, which in turn has a groupBy() method. And DataList has a method called "alterDataQuery" which wants a callback and returs a fresh, new and modified DataQuery (hidden pun for my German speaking readers). So i came up with this solution in my Page_Controller:

public function getPopularBlogTags($limit = 5) {
	$tags = BlogTag::get()
		->setQueriedColumns(['ID', 'Title', 'Count(*)'])
		->leftJoin('BlogPost_Tags','bpt.BlogTagID = BlogTag.ID','bpt')
		->sort('Count(*) DESC')

	return $tags;

and in my template i can now loop over the most popular tags like:

<% cached 'tags-widget', $List('BlogPost').max('LastEdited'), $List('BlogTag').count() %>
    <% loop $PopularBlogTags %>
        <a href="$Link" title="$Title">$Title ($BlogPosts.Count())</a>
        <% if not $Last %> | <% end_if %>
    <% end_loop %>
<% end_cached %>

Thanks to Barry and Mark on IRC that helped me solving this issue.

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