sub prepare_tree {
  my $sth = $dbh->prepare('SELECT * FROM table');
  $sth->execute;

  my $retval = {};
  while (my $hash = $sth->fetchrow_array) {
    push @{ $retval->{ $hash->{parent_id} } }, { %$hash };
  }

  $sth->finish;

  $retval;
}