DBIx::Class::Helper::ResultSet::CorrelateRelationship(3)

NAME
DBIx::Class::Helper::ResultSet::CorrelateRelationship - Easily correlate your ResultSets

SYNOPSIS
 package MyApp::Schema::ResultSet::Author;

 use parent 'DBIx::Class::ResultSet';

 __PACKAGE__->load_components(qw(Helper::ResultSet::CorrelateRelationship));

 sub with_book_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       book_count => $self->correlate('books')->count_rs->as_query
     }
   });
 }

 1;

And then elsewhere, like in a controller:

 my $rows = $schema->resultset('Author')->with_book_count->all;

DESCRIPTION
Correlated queries are one of the coolest things I've learned about for SQL since my initial learning of SQL. Unfortunately they are somewhat confusing. DBIx::Class has supported doing them for a long time, but generally people don't think of them because they are so rare. I won't go through all the details of how they work and cool things you can do with them, but here are a couple high level things you can use them for to save you time or effort.

If you want to select a list of authors and counts of books for each author, you could use "group_by" and something like "COUNT(book.id)", but then you'd need to make your select list match your "group_by" and it would just be a hassle forever after that. The "SYNOPSIS" is a perfect example of how to implement this.

If you want to select a list of authors and two separate kinds of counts of books for each author, as far as I know, you must use a correlated subquery in DBIx::Class. Here is an example of how you might do that:

 package MyApp::Schema::ResultSet::Author;

 use parent 'DBIx::Class::ResultSet';

 __PACKAGE__->load_components(qw(Helper::ResultSet::CorrelateRelationship));

 sub with_good_book_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       good_book_count => $self->correlate('books')->good->count_rs->as_query
     }
   });
 }

 sub with_bad_book_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       bad_book_count => $self->correlate('books')->bad->count_rs->as_query
     }
   });
 }

 1;

And then elsewhere, like in a controller:

 my $rows = $schema->resultset('Author')
   ->with_bad_book_count
   ->with_good_book_count
   ->all;

This assumes that the Book resultset has "good" and "bad" methods.

See "NOTE" in DBIx::Class::Helper::ResultSet for a nice way to apply it to your entire schema.

METHODS

  correlate
 $rs->correlate($relationship_name)

Correlate takes a single argument, a relationship for the invocant, and returns a resultset that can be used in the selector list.

EXAMPLES

  counting CD's and Tracks of Artists
If you had an Artist ResultSet and you wanted to count the tracks and CD's per Artist, here is a recipe that will work:

 sub with_track_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       track_count => $self->correlate('cds')
         ->related_resultset('tracks')
         ->count_rs
         ->as_query
     }
   });
 }

 sub with_cd_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       cd_count => $self->correlate('cds')
         ->count_rs
         ->as_query
     }
   });
 }

 # elsewhere

 my @artists = $artists->with_cd_count->with_track_count->all;

Note that the following will not work:

 sub BUSTED_with_track_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       track_count => $self->related_resultset('cds')
         ->correlate('tracks')
         ->count_rs
         ->as_query
     }
   });
 }

The above is broken because "correlate" returns a fresh resultset that will only work as a subquery to the ResultSet it was chained off of. The upshot of that is that the above "tracks" relationship is on the "cds" ResultSet, whereas the query is for the Artist ResultSet, so the correlation will be "broken" by effectively "joining" to columns that are not in the current scope.

For the same reason, the following will also not work:

 sub BUSTED2_with_track_count {
   my $self = shift;

   $self->search(undef, {
     '+columns' => {
       track_count => $self->correlate('cds')
         ->correlate('tracks')
         ->count_rs
         ->as_query
     }
   });
 }

AUTHOR
Arthur Axel "fREW" Schmidt <frioux+cpan@gmail.com>

COPYRIGHT AND LICENSE
This software is copyright (c) 2015 by Arthur Axel "fREW" Schmidt.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.