I did find out what went wrong, but I don't know whether or how to delete the question so I shall just answer it ;)

contrary to some documentation I read, one should explicitely declare the primary key, I thought it would automatically be the first ( that is what I read in the doc ) in the 'ALL' list. That is not the case. So MyDB::Foo now reads

package MyDB::Foo;
use base MyDB;
__PACKAGE__->table('foo');
__PACKAGE__->columns( PRIMARY =>qw/foo_id/ );
__PACKAGE__->columns( ESSENTIAL =>qw/name uri/ );
);
1;
[download]

and all is well... ;)