[Dbix-class] Designing a system with DBIx::Class

[Matt S Trout]

On Fri, May 01, 2009 at 04:19:32PM -0700, Morgon Hed wrote:
> 
> << Manage the scoping yourself
> 
> Hmmm. 
> 
> First of all many thanks for your reply.
> 
> What I want is a way to "lazily" only load those objects from the database that are actually needed to process a message and so ideally I would like each object to be able to get a ref to another in a uniform way without having to know wether or not the object has already been loaded...
> I was actually thinking about building a little cache that would hand out weak references so that all objects that needed a reference to another object would use the same ref which would then garbage-collected when nobody needs it anymore - I assume this is pretty much what Class::DBI has been doing - is that a bad idea (and if so why)?

Foo has_one bar => Bar

Bar belongs_to foo => Foo

$foo->bar->foo; # bam, circular reference, memory leak

I think I might be able to solve this but it will involve some magic (if I
do, it'll be in DBIC 09 :)
 
> Maybe you could elaborate a little more how you build your object-level as your "Manage the scoping yourself" is not too clear to me (I assume you mean more than simply "make sure you load every object only once").

If you always traverse the graph in the same direction you can generally avoid
loading things more than once anyway - but the other thing is to keep a
hash of objects yourself, and just forcibly destroy them on end of request -

%$_ = () for values %cache;

should forcibly break any circular references created.

But do apply Devel::Leak or Devel::LeakTrace or similar to double check you
got it right.

(and I hope now you're starting to see why we haven't tried to ship such a
thing in core -yet- - it's a much trickier problem than it first appears ...)

-- 
        Matt S Trout         Catalyst and DBIx::Class consultancy with a clue
     Technical Director      and a commit bit: http://shadowcat.co.uk/catalyst/
 Shadowcat Systems Limited
  mst (@) shadowcat.co.uk        http://shadowcat.co.uk/blog/matt-s-trout/