[Dbix-class] RFC: What to do with $resultset->single returning multiple rows?

[John Napiorkowski]

--- On Tue, 5/6/08, Matt S Trout <dbix-class at trout.me.uk> wrote:

> From: Matt S Trout <dbix-class at trout.me.uk>
> Subject: Re: [Dbix-class] RFC: What to do with $resultset->single returning multiple rows?
> To: jjn1056 at yahoo.com, "DBIx::Class user and developer list" <dbix-class at lists.scsys.co.uk>
> Date: Tuesday, May 6, 2008, 1:53 PM
> On Mon, May 05, 2008 at 05:19:01PM -0700, John Napiorkowski
> wrote:
> > Hi,
> > 
> > While trying to track down a bug in .08 trunk that was
> causing an error on mysql I discovered that the problem was
> due to a change in the way
> DBIx::Class::Storage::DBI->select_single handled results
> that returned more than a single row.  In 0.08010 and
> earlier, if ->select_single was run against SQL that
> returned more than a single row, only the first row was
> returned and the remaining ones silently discarded.  On
> DBIC trunk, a bit of code was added that would carp out if
> more rows existed:
> > 
> > sub select_single {
> >   my $self = shift;
> >   my ($rv, $sth, @bind) = $self->_select(@_);
> >   my @row = $sth->fetchrow_array;
> >   carp "Query returned more than one row" if
> $sth->fetchrow_array;
> >   # Need to call finish() to work round broken DBDs
> >   $sth->finish();
> >   return @row;
> > }
> > 
> > 
> > Now, from what I can see, the entire point of this
> method is to help optimise $resultset->find by just
> returning a single row without creating a whole cursor. 
> However, DBIC::Storage::DBI->select_single is also
> publicly exposed as $resultset->single and is documented
> as a optimised select for a single row.  Nothing in the docs
> or previous versions warned users that >single should
> only be run against SQL that returns a single row.  In
> fact, one would think that ->find is reserved for this
> purpose.  So by adding this additional constraint at that
> point I am very certain we are going to be breaking peoples
> code.  I know in fact $resultset->single is used in a few
> spots in our codebase in places that will explode if the
> above carp is kept.
> 
> carp is a warning.
> 
> It's warning you that your code is broken.
> 
> Fix the bug, keep the warning.
> 
> There's no incompatiblity here at all, just that
> we're warning people with
> broken code that they have broken code.
> 
> If your query doesn't return a single result, using
> single() makes no
> sense.
> 
> -- 
>       Matt S Trout       Need help with your Catalyst or
> DBIx::Class project?
>    Technical Director                   
> http://www.shadowcat.co.uk/catalyst/
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