Selection::from_record does not respect scaled

[ctb]

In selection.rs, Selection::from_record does not pull the scaled value in from the Record:

sourmash/src/core/src/selection.rs

Lines 122 to 131 in e86c8a8

	pub fn from_record(row: &Record) -> Result<Self> {
	Ok(Self {
	ksize: Some(row.ksize()),
	abund: Some(row.with_abundance()),
	moltype: Some(row.moltype()),
	num: None,
	scaled: None,
	containment: None,
	picklist: None,
	})

Among perhaps other side effects, this means that Collection::sig_from_record does not return an appropriately downsampled signature.

This confusion was the source of a bug in branchwater - sourmash-bio/sourmash_plugin_branchwater#505.

[ctb]

Interestingly we apparently figured this out before, because @bluegenes noted it in a test 😆

sourmash/src/core/src/collection.rs

Lines 479 to 483 in e86c8a8

	for (idx, _rec) in cl.iter() {
	// need to pass select again here so we actually downsample
	let this_sig = cl.sig_for_dataset(idx).unwrap().select(&selection).unwrap();
	let this_mh = this_sig.minhash().unwrap();
	assert_eq!(this_mh.scaled(), 100);

[ctb]

Tracking it down a bit more, it looks like maybe this is intentional - Collection::select delegates to Manifest::select which merely selects rows with compatible scaled values, but does not actually set the scaled value in the Record.

sourmash/src/core/src/manifest.rs

Lines 280 to 283 in e86c8a8

	valid = if let Some(scaled) = selection.scaled() {
	// num sigs have row.scaled = 0, don't include them
	valid && row.scaled != 0 && row.scaled <= scaled
	} else {

[ctb]

The contortions I'm going through over in sourmash-bio/sourmash_plugin_branchwater#504 make me think that we definitely want to provide a way to request downsampling on sig_from_record 😆

It felt related to sourmash-bio/sourmash_plugin_branchwater#501 to me, too, but after rereading everything I'm not sure. Sigh. Complexities.

[ctb]

per slack, luiz speaketh:

I think this is a straight bug, should be setting scaled and num too!

(later conversation: we're not sure about num ;))

[ctb]

Fixed by #3387.