[Dbix-class] String evaluation in resultset->next

[Christian Lackas]

Hi Everybody,

I frequently use 

    use overload '""' => sub { shift->toString }, fallback => 1;

for my Row classes (via a role). However, recently noticed lots of
warnings about undefined values when I used select/as in a query to only
fetch a subset of columns.

Today, I found some time to actually track this down and found that

    while (my $dp = $resultset->next) {
    }

seems to call my stringify operator (and finally toString). Please note
that this happens even with an empty while loop, so none of my code uses
$dp in a string conext.

The alternative code to fetch all rows of a resultset does not show this
behavior and no stringify is called along the way.

    for my $dp ($resultset->all) {
    }

As said, this is only an issue if next is used on a resultset that does
not provide the usual columns (my toString expects).
However, besides the warnings, I am also wondering if there is a
performance penalty by using stringify without actually needing it.
Also, I believe this next-behavior is also responsible for a 'deep
recursion' error I see in a more complex Template Toolkit usage (at
least I see similar warnings and using a for loop worked around the
issue; have not fully investigated this yet, though).

Is this behavior intentional or a bug?

Unfortunately, I was not able to find the root for it. Adding a 'print
caller' only points back to the '$resultset->next' line.
Please also note that

    my $row = $rs->next;

itself does not trigger the issue. It is somehow related to the while()
loop (which indicates it may not be an issue in DBIx::Class itself?).


Please find attached a small test you can just drop into you DBIx::Class
t folder and run with

    $ prove -Ilib t/stringify.t 
    t/stringify.t .. 1/? 
    #   Failed test 'Stringify called'
    #   at t/stringify.t line 12.
    
    #   Failed test 'Stringify called'
    #   at t/stringify.t line 12.
    
    #   Failed test 'Stringify called'
    #   at t/stringify.t line 12.
    
    #   Failed test 'Stringify called'
    #   at t/stringify.t line 12.
    
    #   Failed test 'Stringify called'
    #   at t/stringify.t line 12.
    # Looks like you failed 5 tests of 6.
    t/stringify.t .. Dubious, test returned 5 (wstat 1280, 0x500)
    Failed 5/6 subtests 
    
    Test Summary Report
    -------------------
    t/stringify.t (Wstat: 1280 Tests: 6 Failed: 5)
      Failed tests:  1-5
      Non-zero exit status: 5
    Files=1, Tests=6,  1 wallclock secs ( 0.02 usr  0.00 sys +  0.29 cusr  0.01 csys =  0.32 CPU)
    Result: FAIL

I tested this with DBIx-Class-0.082810 and freshly updated dependencies.
On our production system we are still using 0.08204. Perl 5.18.2 and
5.16.2.

Christian

-- 
Dr. Christian Lackas, Managing Partner
inviCRO, LLC -- In Imaging Yours
http://www.invicro.com/  http://www.spect-ct.com/
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