more model refinement

MODELS.md

@@ -287,23 +287,14 @@ struct Article {
 A Comment represents a textual comment that a Person wants to attach contextually to an Artist, an Album, a Track, or a Playlist.
 
 ``` rust
-enum CommentTargetType {
-    Label,
-    Artist,
-    Album,
-    Track,
-    Playlist,
-    Article,
-    Comment,
-}
-struct CommentTargetId {
-    label_id: Option<LabelId>,
-    artist_id: Option<ArtistId>,
-    album_id: Option<AlbumId>,
-    track_id: Option<TrackId>,
-    playlist_id: Option<PlaylistId>,
-    article_id: Option<ArticleId>,
-    comment_id: Option<CommentId>,
+enum CommentTarget {
+    Label(LabelId),
+    Artist(ArtistId),
+    Album(AlbumId),
+    Track(TrackId),
+    Playlist(PlaylistId),
+    Article(ArticleId),
+    Comment(CommentId),
 }
 struct CommentId {
     id: String,
@@ -311,8 +302,7 @@ struct CommentId {
 struct Comment {
     id: CommentId,
     body: String,
-    target_type: CommentTargetType,
-    target_id: CommentTargetId,
+    target: CommentTarget,
     created_by: PersonId,
     created_at: chrono::DateTime,
     modified_by: Option<PersonId>,
@@ -436,10 +426,10 @@ A Purchase represents one or more Tracks or Albums that a Person purchases from
 
 ``` rust
 enum PurchaseItemType {
-    Album,
-    Track,
-    Collection,
-    OtherProduct,
+    Album(AlbumId),
+    Track(TrackId),
+    Collection(CollectionId),
+    OtherProduct(OtherProductId),
 }
 struct PurchaseItemId {
     id: String,
@@ -449,10 +439,7 @@ struct SkuId {
 }
 struct Sku {
     id: SkuId,
-    album_id: Option<AlbumId>,
-    track_id: Option<TrackId>,
-    collection_id: Option<CollectionId>,
-    other_product_id: Option<OtherProductId>,
+    item: PurchaseItemType,
     variant: Option<String>,
     description: Option<String>,
     price: Option<rusty_money::Money>,
@@ -460,8 +447,7 @@ struct Sku {
 }
 struct PurchaseItem {
     id: PurchaseItemId,
-    type: PurchaseItemType,
-    sku: Option<SkuId>,
+    sku: SkuId,
     quantity: usize,
 }
 enum PurchaseState {