Add track number to ID3 metadata

I've reused the formatted track position that was already being used for
the filename prefix in `ZipDownloadJob#track_filename`, because I
thought it was worth being consistent.

We're using ID3v2.3 and it seems as if the `TRCK` tag is the one we
want. See the earlier commit [1] where we added the other metadata for
more details. As in that commmit, `Track#number` will never be blank, so
there's no need to add it to the metadata hash individually.

We'll need to re-transcode all tracks in order to apply this to existing
tracks, but I suggest we tackle that as a separate manual task where
we'll need to run `Artist.all.each(&:transcode_albums)` in a Rails
console.

[1]: 1851e27

app/jobs/zip_download_job.rb

@@ -46,7 +46,7 @@ def download_all_tracks(album, format, dir)
   end
 
   def track_filename(track, format)
-    Zaru.sanitize!("#{track.position.to_s.rjust(2, '0')} - #{track.title}.#{extension(format)}")
+    Zaru.sanitize!("#{track.number} - #{track.title}.#{extension(format)}")
   end
 
   def extension(format)

app/models/track.rb

@@ -29,6 +29,7 @@ def preview_duration
   def metadata
     {
       track_title: title,
+      track_number: number,
       album_title: album.title,
       artist_name: artist.name
     }
@@ -43,4 +44,8 @@ def transcode
   def metadata_or_original_changed?
     title_previously_changed? || attachment_changes['original'].present?
   end
+
+  def number
+    position.to_s.rjust(2, '0')
+  end
 end

app/models/transcode_command.rb

@@ -3,6 +3,7 @@
 class TranscodeCommand
   METADATA_KEYS_VS_ID3V23_TAGS = {
     track_title: 'TIT2',
+    track_number: 'TRCK',
     album_title: 'TALB',
     artist_name: 'TPE1'
   }.freeze

test/models/track_test.rb

@@ -90,7 +90,14 @@ class TrackTest < ActiveSupport::TestCase
     metadata = track.metadata
 
     assert_equal track.title, metadata[:track_title]
+    assert_equal track.number, metadata[:track_number]
     assert_equal track.album.title, metadata[:album_title]
     assert_equal track.album.artist.name, metadata[:artist_name]
   end
+
+  test '#number' do
+    album = create(:album_with_tracks)
+
+    assert_equal %w[01 02], album.tracks.map(&:number)
+  end
 end

test/models/transcode_command_test.rb

@@ -43,11 +43,17 @@ class TranscodeCommandTest < ActiveSupport::TestCase
   end
 
   test 'adds metadata using ID3v2.3 format' do
-    metadata = { track_title: 'track-title', album_title: 'album-title', artist_name: 'artist-name' }
+    metadata = {
+      track_title: 'track-title',
+      track_number: 'track-number',
+      album_title: 'album-title',
+      artist_name: 'artist-name'
+    }
     command_string = TranscodeCommand.new(@input, @output, :mp3v0, metadata).generate
     assert_contains_pair(command_string, ['-write_id3v2', '1'])
     assert_contains_pair(command_string, ['-id3v2_version', '3'])
     assert_contains_pair(command_string, ['-metadata', 'TIT2="track-title"'])
+    assert_contains_pair(command_string, ['-metadata', 'TRCK="track-number"'])
     assert_contains_pair(command_string, ['-metadata', 'TALB="album-title"'])
     assert_contains_pair(command_string, ['-metadata', 'TPE1="artist-name"'])
   end