chore: Docs, define additional client-side relations

app/models/Subscription.ts

@@ -1,22 +1,28 @@
 import { observable } from "mobx";
+import Document from "./Document";
+import User from "./User";
 import Model from "./base/Model";
 import Field from "./decorators/Field";
+import Relation from "./decorators/Relation";
 
 /**
- * A subscription represents a request for a user to receive notifications for
- * a document.
+ * A subscription represents a request for a user to receive notifications for a document.
  */
 class Subscription extends Model {
-  @Field
-  @observable
-  id: string;
-
-  /** The user subscribing */
+  /** The user ID subscribing */
   userId: string;
 
-  /** The document being subscribed to */
+  /** The user subscribing */
+  @Relation(() => User, { onDelete: "cascade" })
+  user?: User;
+
+  /** The document ID being subscribed to */
   documentId: string;
 
+  /** The document being subscribed to */
+  @Relation(() => Document, { onDelete: "cascade" })
+  document?: Document;
+
   /** The event being subscribed to */
   @Field
   @observable