package Obj;

use constant PI => 3.14;
use constant  G => 9.81;

my %foo_cache;
my %bar_cache;

use parent ('Obj::Foo', 'Obj::Bar');

sub new {
  my ($class, $dbh, $id) = @_;

  my $self = bless {}, $class;
  $self->{dbh} = $dbh;
  $self->{id} = $id;
  
  return $self;
}

sub dbh {
  my $self = shift;
  return $self->{dbh};
}

sub id {
  my $self = shift;
  return $self->{id};
}

1;

###############
package Obj::Foo;

sub foo {
  my $self = shift;
  my $id   = $self->id;
  my $dbh  = $self->dbh;

  unless ($self->{foo}) {
  
    # check if foo is in cache
    if (exists $Obj::foo_cache{$id}) {
      # return from cache
      $self->{foo} = $Obj::foo_cache{$id};
    }
    else {
      $Obj::foo_cache{$id} = Obj::PI * query_foo_from_db_using($dbh, $id);
      $self->{foo} = $Obj::foo_cache{$id};
    }
  }
  
  return $self->{foo};
}

1;