NAME¶
DBIx::Class::Helper::ResultSet::CorrelateRelationship - Easily correlate your
ResultSets
SYNOPSIS¶
package MyApp::Schema::ResultSet::Author;
use base '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 base '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) 2014 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.