[Dbix-class] Implicit prefetch (via select/columns) not supported with accessor 'multi'

[Christiaan Kras]

Op 16-4-2011 19:01, Robert Kinyon schreef:
> On Sat, Apr 16, 2011 at 10:58, Christiaan Kras<c.kras at pcc-online.net>  wrote:
>> Hello list,
>>
>> I've been banging my head against the wall because of an error message I'm
>> getting which I wasn't getting before. I'm now using DBIx::Class 0.08127,
>> whilst before I believe it was 0.08123.
>>
>> I'm constructing my search with the following data:
>>
>>     my @search = (
>>         {   'modscope.name'           =>  $modscope,
>>             'me.name'                 =>  $module,
>>             'module_versions.version' =>  $version,
>>         },
>>         {
>>             join       =>  [qw /modscope module_versions/],
>>             '+columns' =>  ['module_versions.remote_location'],
>>         }
>>     );
>>
>>     my $rs = $c->model('DB::Module')->search(@search);
>>     my $row = $rs->first;
>>     ...
>>
>> This generates the correct query for my case. The reason I explicitly tell
>> it to add the module_versions.remote_location column is because the
>> module_versions tables contains a BLOB column that can be big. If the
>> remote_location appears to be empty I'll fetch it in a new call, otherwise
>> it redirects the user to the remote_location (this is a Catalyst
>> application).
>>
>> In 0.08123 and 0.08124 this works as I intended and described.
>>
>> In 0.08125 and 0.08127 it doesn't and I'm presented with this error message:
>> "Implicit prefetch (via select/columns) not supported with accessor
>> 'multi'". The moment the database call is being made ($rs->first) the given
>> error message shows up.
>>
>> My question is, is this a bug in DBIx::Class or am I doing something that
>> wasn't supposed to work from the beginning?
>>
>> If this behavior is intended could one give me directions on how to refactor
>> my code? I'm also curious as why this change was made if this is indeed the
>> case.
>
> The old behavior was a bug. What is happening is that you have a
> Module->has_many(ModuleVersions). When the +columns happens, DBIC
> doesn't know that there is only one possible value for
> module_versions.remote_location. Your query may be able to guarantee
> this, but DBIC doesn't know that when it's building the query.
>
> The simplest fix is to remove the +columns, then traverse the
> relationship. When you do that, you'll see exactly why DBIC cannot
> assume anything.
>
> Alternately, prefetch module_versions. :)
>

OK, that makes sense.

With prefetching am I able to exclude a column from the query? Like I 
said the reason I was doing this was because one of the columns is a 
BLOB. Otherwise I should perhaps look at storing this data somewhere 
else (perhaps another dedicated table).

-- 
Christiaan Kras
http://blog.htbaa.com