Additional conditions in JOIN - perl

Question

I have tables with articles and users, both have many-to-many mapping to third table - reads.
What I am trying to do here is to get all unread articles for particular user ( user_id not present in table reads ).
My query is getting all articles but those read are marked, which if fine as I can filter them out (user_id field contains id of user in question).
I have an SQL query like this:
SELECT articles.id, reads.user_id
FROM articles
LEFT JOIN
reads
ON articles.id = reads.article_id AND reads.user_id = 9
ORDER BY articles.last_update DESC LIMIT 5;
Which yields following:
articles.id | reads.user_id
-------------------+-----------------
57125839 | 9
57065456 |
56945065 |
56945066 |
56763090 |
(5 rows)
This is fine. This is what I want.
I'd like to get same result in Catalyst using my article model, but I cannot find any option to add conditions to a JOIN clause.
Do you know any way how to add AND X = Y to DBIx JOIN?
I know this can be done with custom resoult source and virtual view, but I have some other queries that could benefit from it and I'd like to avoid creating virtual view for each of them.
Thanks,
Canto

Answer 1

I don't even know what Catalyst is but I can hack the SQL query:
select articles.id, reads.user_id
from
articles
left join
(
select *
from reads
where user_id = 9
) reads on articles.id = reads.article_id
order by articles.last_update desc
limit 5;

Answer 2

I got an solution.
It's not straight forward, but it's better than virtual view.
http://search.cpan.org/dist/DBIx-Class/lib/DBIx/Class/Relationship/Base.pm#condition
Above describes how to use conditions in JOIN clause.
However, my case needs an variable in those conditions, which is not available by default in model.
So getting around a bit of model concept and introducing variable to it, we have the following.
In model file
our $USER_ID;
__PACKAGE__->has_many(
pindols => "My::MyDB::Result::Read",
sub {
my $args = shift;
die "no user_id specified!" unless $USER_ID;
return ({
"$args->{self_alias}.id" => { -ident => "$args->{foreign_alias}.article_id" },
"$args->{foreign_alias}.user_id" => { -ident => $USER_ID },
});
}
);
in controller
$My::MyDB::Result::Article::USER_ID = $c->user->id;
$articles = $channel->search(
{ "pindols.user_id" => undef } ,
{
page => int($page),
rows => 20,
order_by => 'last_update DESC',
prefetch => "pindols"
}
);
Will fetch all unread articles and yield following SQL.
SELECT me.id, me.url, me.title, me.content, me.last_update, me.author, me.thumbnail, pindols.article_id, pindols.user_id FROM (SELECT me.id, me.url, me.title, me.content, me.last_update, me.author, me.thumbnail FROM articles me LEFT JOIN reads pindols ON ( me.id = pindols.article_id AND pindols.user_id = 9 ) WHERE ( pindols.user_id IS NULL ) GROUP BY me.id, me.url, me.title, me.content, me.last_update, me.author, me.thumbnail ORDER BY last_update DESC LIMIT ?) me LEFT JOIN reads pindols ON ( me.id = pindols.article_id AND pindols.user_id = 9 ) WHERE ( pindols.user_id IS NULL ) ORDER BY last_update DESC: '20'
Of course you can skip the paging but I had it in my code so I included it here.
Special thanks goes to deg from #dbix-class on irc.perl.org and https://blog.afoolishmanifesto.com/posts/dbix-class-parameterized-relationships/.
Thanks,
Canto