How use field value as key in path?

[bpj]

What’s the correct way to use a mapping value as a key in a path?

I have a file foo.yaml loaded into a variable $foo and want to update values in the current object so that I use the value of a field in the current object as a key in the path to the update value, i.e. what I in Perl would do with

$_->{num} //= $foo->{stuff}{ $_->{str} // 'null' }{num};

I thought the yq syntax for this would be

.num = ( .num // $foo.stuff.[.str].num )

but the .[.str] bit doesn’t seem to work; .num ends up as null.

It works if I replace it with the hardcoded key.

Note: Hash (mapping) keys must be strings in Perl so the dummy field with key null in becomes 'null'. One of the reasons I want to do this with yq.