DBIx::Class Tricks: the not-so-scalar ResultSet
I tend to write bulk loaders that work in batches: I gave the application an amount of work to do, and then expect that the application does exactly that amount of work an no more. Except when the application decides to do more work! The scenario is like the following one:- there is a table where every entry has a
validfield, that if false means that the entry has to be worked out (whatever it means to you); - for every entry worked out, decrease the original batch size by one unit and continue;
- exit or stop when the batch size hits zero.
DBIx::Class query can ruin the day!
my $count_entries_to_do = $db->resultset( 'Entry' )->search( { valid => 0 } );
if ( $count_entries_to_do > 0 ) {
# do stuff on all the entries
# ...
# and remove them from the batch size
$batch_size -= $count_entriess_to_do; # argh !
}
Can you see the problem with the above piece of code? Well,
search returns a DBIx::Class::Resultset, which in scalar context evaluates to the tuple count, but this does not mean it is a scalar value!
So, whenever you evaluate the scalar variable associated to the DBIx::Class::ResultSet object, DBIx::Class will perform a new query, so changing the count.
Imagine you have 20 entries, and the batch size is set to 20: you are expecting that at the end the expression $batch_size -= $count_entries_to_do will evaluate to 0, but in reality the expression will be evaluated to 20 because the query will run again, finding no valid => 0 entries.
This is not immediatly understandable, because if you set the batch size to 10 and the total number of entries to be processed is 20, you will end up with the expected result of 10 remaining, but next runs will raise the problem.
Clearly, this is a super power feature of DBIx::Class, but it does not work out well in this scenario!
The solution, obviously, is to say to DBIx::Class that we need effectively the counting, so a scalar value, not an object that can evaluate to a scalar:
my $count_entries_to_do = $db->resultset( 'Entry' )->count( { valid => 0 } );
if ( $count_entries_to_do > 0 ) {
# do stuff on all the entries
# ...
# and remove them from the batch size
$batch_size -= $count_entriess_to_do; # ok!
}