[Tracker] Some nmm:MusicAlbum issues

[MichaÅ Sawicz <michal sawicz net>]

Hi everyone.

I have talked a bit with pvanhoof, ottela and abustany about some issues
I've come into when dealing with music albums in Tracker and we decided
that they need to be discussed more widely.

1. The nmm:albumArtist isn't handled at all, and that's what I intend to
fix in the long run. The TPE2 / TP2 frames in ID3v2 and friends are
widely used as an album-artist tag for 'compilation' albums, and that
tag should be reflected in the nmm:albumArtist of nmm:MusicAlbum. Philip
is says this should only be set when the album-artist tag is present.
This approach has the added difficulty of requiring a query for
nmm:MusicPiece when determining the artist name of an album that does
not have a nmm:albumArtist. On the other hand this allows to distinguish
compilation albums that are not otherwise marked as such (there is a
'compilation' tag in ID3v2, but it's not used as widely).

2. Currently albums have uris created with 'urn:album:%s' where %s is
replaced with the album title. This causes several issues, mostly coming
down to that uri not being unique. Every nmm:MusicPiece of any 'The Best
Of' or 'Greatest Hits' albums, for example, will be linked to a single
nmm:MusicAlbum entity. So distinguishing a separate album requires
grouping by both nmm:musicAlbum and nmm:performer. My proposal is to
base the uri off of nmm:albumArtist if set, or nmm:performer otherwise,
and nmm:albumTitle. These should probably be normalized somehow (at
least lowercased). Adrien suggested the use of UUIDs, Mikael noticed
this would require searching through existing nmm:MusicAlbums just to
get it's urn. The UUID could, on the other hand, be based on the same
properties as above, we just need a coherent scheme.

3. Multi-disc albums. Already filed as bgo#621430, nmm:setNumber is
currently a property of nmm:MusicAlbum. It's not being set by the
extractors, either, but if it were - this scheme would require a copy of
nmm:MusicAlbum for each disc in a multidisc album. The nmm:setNumber
should either be moved to nmm:MusicPiece or maybe there needs to be an
intermediate resource between nmm:MusicAlbum and nmm:MusicPieces.

4. Release dates, again, are only written to nmm:MusicPieces (to
nie:contentCreated), whereas nmm:MusicAlbums could have this set, too.
This would, probably, contain exactly the same data in 99.999% cases,
but IMO requires some thought put into it as well.

Mikael also mentioned that these are all TODO entries, along with some
duration considerations. I think, if required, the extractors could sum
up nmm:MediaPieces' durations after each relevant file was scanned, but
I think the duration of an album is not necessarily identical to that
sum. The easiest example is when you have missing tracks. The album
duration, and track count as well, should IMO be taken from an external
source, if possible.

That seems to be all I've found ATM. I can provide relevant IRC logs to
anyone interested.

Best regards
-- 
MichaÅ (Saviq) Sawicz <michal sawicz net>