Subclassing Curiosity

[Seb]

I have a curiosity with regard to subclassing DBI. I'm just not sure I
get the principles involved. I've got a module that is a DBI subclass.
In order to subclass DBI as "package Mine", one must also provide
packages Mine::db and Mine::st, because the database and statement
handles used in DBI are their own classes.

My confusion comes in when I put a constructor in my main package, and
try to use DBI methods on its returned object. (DBI has no constructor
method with which to reference a DBI object, per se)

So I have something like this:

#!/usr/bin/perl
package MyDBI;
use DBI;
@ISA = qw(DBI);

sub new {
  my $class = shift;
  my $self = {};
  bless $self, $class;
  return $self;
}

package MyDBI::db;
@ISA = qw(DBI::db);

package MyDBI::st;
@ISA = qw(DBI::st);

# now our main program

package main;

my $obj = new MyDBI;

# $obj is a MyDBI object, with DBI base methods
print '$obj is a: '.(scalar ref $obj)."\n";

my $dbh = $obj->connect('DBI:Pg:dbname=somedb', undef, undef);

# This generates a warning
# $dbh is a DBI::db object, NOT a MyDBI::db object
print '$dbh is a: '.(scalar ref $dbh)."\n";

# On the other hand, if I use the module name, not my object...

my $dbh2 = MyDBI->connect('DBI:Pg:dbname=somedb', undef, undef);

# No error, and $dbh2 is a proper MyDBI::db object
print '$dbh2 is a: '.(scalar ref $dbh2)."\n";

__END__

The output of the above code looks like this:
$obj is a: MyDBI
DBI subclass 'MyDBI=HASH(0x813c1a0)::db' isn't setup, ignored at MyDBI.pl line 26
$dbh is a: DBI::db
$dbh2 is a: MyDBI::db
<disconnect errors snipped>

Why does the first case ($obj->connect) not do what I expect it to,
while the second case (MyDBI->connect) works fine?

Is this just some basic scoping or inheritance thing that I'm not
understanding?

TIA
Sebastian