[OT [OT]] - assering database query results

Joel Bernstein joel at fysh.org
Thu Sep 17 14:32:53 BST 2009

2009/9/17 Nicholas Clark <nick at ccl4.org>:
> Off topicly off topic, as it's not even about that 4 letter P word.
> So, we have quite a lot of our database queries in config files, with
> placeholders, etc
> Some of them are only supposed to return 1 row.
> However, if our assumptions are wrong, they might return multiple rows.
> This isn't a fatal bug in our code, which is written such that it takes the
> first row returned. However, it is a bug in our logic.
> Is there a good way to "assert" that there is only one row? So that in the
> development (and QA) environments, it is fatal to breach assumptions.
> But production keeps going.

DBIx::Class::Storage::DBI has:
sub select_single {
  my $self = shift;
  my ($rv, $sth, @bind) = $self->_select(@_);
  my @row = $sth->fetchrow_array;
  my @nextrow = $sth->fetchrow_array if @row;
  if(@row && @nextrow) {
    carp "Query returned more than one row.  SQL that returns multiple
rows is DEPRECATED for ->find and ->single";
  # Need to call finish() to work round broken DBDs
  return @row;

which is what $schema->resultset("Blah")->search(...)->single hits.

You could reasonably turn that carp into a croak. However, if you
meant to model this assertion at DB-level rather than in your
consuming application's code, I'm out of ideas.


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