[Maypole] Maypole model inheritance

[Dave Howorth]

Simon Flack wrote:
> Dave Howorth wrote:
>> Now it's morning, I see an obvious evil patch that's a variation of
>> my experiment last night. Simply start with the original M-M-CDBI
>> that *does* inherit from C-DBI so that all the C-DBI-* modules can
>> be used. Then at the end of setup_database(), once the
>> loader-determined C-DBI subclass is known, _remove_ C-DBI from
>> M-M-CDBI's @ISA and substitute the C-DBI subclass.
> 
> Unfortunately, that'll break under mod_perl if you have one app using
> a Mysql datasource and another using some other datasource such as Pg
> or SQLite.

A variant would be to leave C-DBI in place in M-M_CDBI's @ISA but push 
it rather than unshift it onto the table subclasses' @ISA. I still 
haven't looked into why an unshift was used ...  (See below why you 
might want to do this).

> One solution is to remove Class::DBI entirely from M::M::CDBI's @ISA
> and make your table classes inherit from a new base class. Consider
> the following inheritance model:
> 
> @MyApp::Model::Table::ISA = (MyApp::Model::_CDBI) 
> @MyApp::Model::_CDBI::ISA = (M::M::CDBI, Class::DBI::$driver) 
> @Maypole::Model::CDBI::ISA = (M::M::Base)
> 
> M::M::CDBI can defer loading of Class::DBI::RetrieveAll, etc until
> the new base class is created. And $r->config->model remains near the
> front of your table classes' @ISA, where it belongs.

The difficulty here is that it's C-DBI-Loader that makes 
MyApp::Model::Table inherit directly from Class::DBI::$driver. So to 
make this work, you have to rewrite C-DBI-Loader, I think? Which seems 
odd, since the scenario (using C-DBI-Loader to set up the table classes 
but also wanting to use other C-DBI-* modules) is pure C-DBI, no 
Maypole, so there ought to be an existing solution out there somewhere.

OTOH, if you're willing to discard C-DBI-Loader, my auto-generated 
application class doesn't suffer from this problem (It's MySQL only at 
the moment, but it doesn't have the problem in principle :)

Also, mightn't there be another problem with your suggested solution if 
any of the C-DBI-* modules initialize differently when connected to 
different databases? The only safe place for them to be loaded would be 
MyApp::Model::_CDBI (or the individual table classes).

Cheers, Dave