sub _select_fields
    {
        my $self = shift;
        my %rev_types = (
            both    => sub { 
                # some difficult logic
            },
            total   => sub { [qw/foo bar/] },
        );

        my $rev_type = $self->request->param('rev_type');
        $rev_type    = 'both' unless exists $rev_types{$rev_type};
        return @{$rev_types{$rev_type}->()};
    }