Hi all, I am seeking wisdom from those experienced in the deep magic of DBIx::Class. What I need to do is return a subset of rows from an SQL query, being the top N results in each group.

This problem is actually part of adding a feature to the CPAN Testers API, but for the sake of argument, let's use the standard DBIC metaphor of a music collection. Having started with all CDs, and filtered for musical genre, I want to receive only the most recent N albums by each artist in the result set.

In SQL I can do this with row_number(), or, for MySQL, a workaround with user variables. Something like: 

set @artist = '';
set @num = 1;
select * from (
    select c.name,
           c.artist,
           @num := if(@artist = c.artist, @num + 1, 1) as row_number,
           @artist := s.artist as dummy
    from cds as c
) as results
where results.row_number <= 5;

[download]

With DBIC I have already produced a result set spec (but not queried the DB yet) with something like: 

my $rs = $schema->resultset('cds');
$rs = $rs->search({ genre => 'rock' });

[download]
... and I now would like to do something like: 
$rs = $rs->limit_per_artist( 5 );

[download]
... to retrieve the most recent 5 CDs for each rocker.

I can see a couple of different ways to do it that involve multiple passes over the table(s), but I really need a single-pass solution like the SQL shown. Any pointers much appreciated.


The way forward always starts with a minimal test.

In reply to Get top N rows in each group with DBIx::Class by 1nickt

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