Render useful errors when album artists are invalid

app/controllers/albums_controller.rb

@@ -86,8 +86,18 @@ def transformed_attributes
     end
 
     if attributes[:album_artists].present?
-      attributes[:album_artists] = attributes[:album_artists].map do |aa, _i|
-        AlbumArtist.new(artist_id: aa[:artist_id], name: aa[:name], separator: aa[:separator], order: aa[:order] || 0)
+      prev_aa = @album&.album_artists.to_a
+      attributes[:album_artists_attributes] = attributes.delete(:album_artists).map do |aa|
+        existing = prev_aa.find { it.artist_id == aa[:artist_id] }
+        unless existing.nil?
+          prev_aa.delete(existing)
+          aa.merge!({ id: existing.id })
+        end
+        aa
+      end
+      prev_aa.each do |aa|
+        attributes[:album_artists_attributes].push({ id: aa.id, _destroy: true })
+        attributes[:album_artists_attributes].last.permit!
       end
     end
 

app/models/album.rb

@@ -37,6 +37,8 @@ class Album < ApplicationRecord
   has_many :playlists, through: :playlist_items, source: :playlist
   belongs_to :image, optional: true, dependent: :destroy
 
+  accepts_nested_attributes_for :album_artists, allow_destroy: true
+
   before_validation :normalize_artist_order
 
   validates :title, presence: true

test/controllers/albums_controller_test.rb

@@ -130,6 +130,31 @@ class AlbumsControllerTest < ActionDispatch::IntegrationTest
     assert_response :created
   end
 
+  test 'should not create album and render errors if invalid' do
+    sign_in_as create(:moderator)
+    attributes = attributes_for(:album)
+
+    assert_no_difference 'Album.count' do
+      post albums_url, params: { album: { release: attributes[:release], title: '' } }
+    end
+
+    assert_response :unprocessable_content
+    assert_includes response.parsed_body['errors'], { 'model' => 'album', 'attribute' => 'title', 'type' => 'blank' }
+  end
+
+  test 'should not create album and render errors if album artists are invalid' do
+    sign_in_as create(:moderator)
+    artist = create(:artist)
+    attributes = attributes_for(:album)
+
+    assert_no_difference 'Album.count' do
+      post albums_url, params: { album: attributes.merge({ album_artists: [{ artist_id: artist.id, name: '', order: 1 }] }) }
+    end
+
+    assert_response :unprocessable_content
+    assert_includes response.parsed_body['errors'], { 'model' => 'album', 'attribute' => 'album_artists.name', 'type' => 'blank' }
+  end
+
   test 'should show album' do
     get album_url(@album)
 
@@ -197,6 +222,44 @@ class AlbumsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
   end
 
+  test 'should update album and album artists' do
+    sign_in_as create(:moderator)
+    album_artist = create(:album_artist, album: @album, separator: nil)
+
+    assert_no_difference '@album.album_artists.reload.count' do
+      assert_changes 'album_artist.reload.name', to: 'New name' do
+        patch album_url(@album), params: { album: { album_artists: [{ artist_id: album_artist.artist_id, name: 'New name' }] } }
+      end
+    end
+
+    assert_response :success
+  end
+
+  test 'should not update album and render errors if invalid' do
+    sign_in_as create(:moderator)
+    album = create(:album)
+
+    assert_no_difference 'Album.count' do
+      patch album_url(album), params: { album: { title: '' } }
+    end
+
+    assert_response :unprocessable_content
+    assert_includes response.parsed_body['errors'], { 'model' => 'album', 'attribute' => 'title', 'type' => 'blank' }
+  end
+
+  test 'should not update album and render errors if album artists are invalid' do
+    sign_in_as create(:moderator)
+    album = create(:album)
+    artist = create(:artist)
+
+    assert_no_difference 'Album.count' do
+      patch album_url(album), params: { album: { album_artists: [{ artist_id: artist.id, name: '', order: 1 }] } }
+    end
+
+    assert_response :unprocessable_content
+    assert_includes response.parsed_body['errors'], { 'model' => 'album', 'attribute' => 'album_artists.name', 'type' => 'blank' }
+  end
+
   test 'should destroy previous image when image is replaced' do
     sign_in_as create(:moderator)
     album = create(:album, :with_image)