my ($table, $key) = split(/__/, $fk);
push @where, "
( $fk IS NOT NULL AND NOT EXISTS(SELECT 1 FROM $table WHERE $table.$key = $fk) )";
my ($table, $key) = split(/__/, $fk);
push @where, "
( $fk IS NOT NULL AND NOT EXISTS(SELECT 1 FROM $table WHERE $table.$key = $fk) )";