[DaveBlake]

Votes -> FYI it works for albums, just not for songs. I'll remove the votes field on my end and add it back when you have fixed it.

Yeap.
I feel very ambivalent about votes for song ratings. Originally songs had a rating out of 5, read from tags. To somewhat arbitrarily "make it like video" this was converted to out of 10, a user rating field and a votes field added. User rating can be set via GUI, but only since v18 are the values exportable so can be kept between installations otherwise it is just transient data and a great waste of user effort. No facility was added to actually set votes (no GUI, songs don't have nfo files, not in JSON, not in embedded tags). Having 2 "ratings" fields has caused users no end of confusion, as did the change in max value of the original. But hey we are consistent with video and that us what matters, right?

Votes for song ratings - is there such data out there? If there is a source of such data and user requirement to use it then a facility to set it can be added of course, meanwhile it makes more sense to be to remove the votes field from the song table (if I change anything, and I'm not rushing at it). Removing the votes field on your end makes the most sense while we wait for users to request the facility (or not).

Artist labels -> Yeah, I've noticed the artistid and artistalbumid table. I'll remove it. I mean, I could update it for all other items as well, but with the limited JSON cmds that will be a pita and cause very loooong processing times.

What are you talking about here @sualfred ?
AudioLibrary.SetXXXDetails does not let you set artist/album/song ids, that would be bonkers! The setting of Musicbrainz ids is acceptable, not sure in what way you would have to update for all items or take a long processing time? The db indexing will reject duplicates, but you don't see that in the JSON results.

However setting mbids is unlikely as something a user would want to do manually. Better that they tag their music using Picard with matching pairs of names and mbids consistenly throughout an album than try to botch it in manually later. It is more likely to get users in a mess than anything helpful.

Note: when JSON returns "OK" for setting call, this just means that the request had correct syntax and was received. It does not mean that the data update was succesfull. This is common across much of the API methods, few actions are sychronous. "OK" is "Kodi got the request and understood it", nothing more.

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@sualfred what is the purpose of this addon? My assumption is that you are making a nice tool to make it easier for users to make the library data additions/edits that they otherwise would have to do via manually editing NFO files. Do I have that right?

Music lib differs from video lib in some fundamental ways - folder structure and file naming is irrelevant, the and the library contents (songs, albums and artists) are created by processing the tag metadata read from the music files themselves. While you can recreate your video library from exported NFO files (at least I think you can), the export of music lib only captures the additional data that can be subsequently reapplied to a new library created from scanning the music files, it does not make the library.

While artists and albums can have NFO files and art providing data in addition to that formulated from the music files, songs do not. The JSON API provides a means to modify song data, but I do have doubts about how helpful it is to enable the user to do that other than for the song playcount, lastplayed and userrating values - those things that can change during use and are (v18 onwards) possible to export/import.

Genres, moods, etc -> How is Kodi handling the audio library scan? Does it rescan already added songs or does it just handle new ones and already added items have to be refreshed manually?

Library update checks folders/files for changes to a hash of size and timestamp and then rescans the contents and deletes existsing db entries prior to inserting new records. Edit the tags of a music file and the next library update it will be found and rescanned. Otherwise in the main if the files does not change it does not get rescanned untill the user does a fresh installation of Kodi, but odd things can change the folder hashes (clock change, or other software touching folder/files adding other files etc.) and sometimes things do get rescanned unexpectedly. Or users rearrange their files etc., drop a source and add it. Changes made to song data by JSON are not exportable to NFO or XML.

I guess the most popular song property to change will be genre - user tagged genre data is messy sometimes and they could want to slip in an correct something they mis-tagged. But it would be so much better to just fix the tags, otherwise if they do ever rescan their music (and there is a good chance that some time they will) then they will have to make those manual changes using your addon all over again.

Lack of permanence of changes to song data isn't my only concern. Changing things like artist(s), track number, title, year, etc. are pretty fundamental to the song identity. If they were wrong in the tagging then what kind of jumble is the library?

 But in the end it is up to you