Make mb_albumartistids available at the album level. (#4909)

As this is a field related to albums, it probably should be accessible
as `album.mb_albumartistids`.

Author: snejus

beets/library.py

@@ -1179,6 +1179,7 @@ class Album(LibModel):
         "comp": types.BOOLEAN,
         "mb_albumid": types.STRING,
         "mb_albumartistid": types.STRING,
+        "mb_albumartistids": types.MULTI_VALUE_DSV,
         "albumtype": types.STRING,
         "albumtypes": types.SEMICOLON_SPACE_DSV,
         "label": types.STRING,
@@ -1235,6 +1236,7 @@ class Album(LibModel):
         "comp",
         "mb_albumid",
         "mb_albumartistid",
+        "mb_albumartistids",
         "albumtype",
         "albumtypes",
         "label",

test/test_importer.py

@@ -129,6 +129,11 @@ def test_import_with_delete_prunes_directory_empty(self):
         self.run_asis_importer(delete=True)
         self.assertNotExists(os.path.join(self.import_dir, b"album"))
 
+    def test_album_mb_albumartistids(self):
+        self.run_asis_importer()
+        album = self.lib.albums()[0]
+        assert album.mb_albumartistids == album.items()[0].mb_albumartistids
+
     @unittest.skipUnless(_common.HAVE_SYMLINK, "need symlinks")
     def test_import_link_arrives(self):
         self.run_asis_importer(link=True)