Ensure that only one transcode of each format exists for each track

Adding this uniqueness constraint will allow us to be a bit less
defensive in the code that looks up transcodes.

app/models/transcode.rb

@@ -4,4 +4,6 @@ class Transcode < ApplicationRecord
   belongs_to :track
   enum format: { mp3v0: 0 }
   has_one_attached :file
+
+  validates :format, presence: true, uniqueness: { scope: [:track_id] }
 end

db/migrate/20231013145058_add_unique_index_on_format_and_track_to_transcode.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddUniqueIndexOnFormatAndTrackToTranscode < ActiveRecord::Migration[7.0]
+  def change
+    add_index :transcodes, %i[format track_id], unique: true
+  end
+end

test/models/transcode_test.rb

@@ -6,4 +6,18 @@ class TranscodeTest < ActiveSupport::TestCase
   test 'fixture is valid' do
     assert build(:transcode).valid?
   end
+
+  test 'validates only one of each format exists for each track' do
+    track = create(:track)
+    different_track = create(:track)
+    create(:transcode, format: :mp3v0, track:)
+
+    duplicate_transcode = build(:transcode, format: :mp3v0, track:)
+
+    assert_not duplicate_transcode.valid?
+
+    different_track_transcode = build(:transcode, format: :mp3v0, track: different_track)
+
+    assert different_track_transcode.valid?
+  end
 end