redesign of createNote APIs, WIP

src/entities/attribute.js

@@ -79,6 +79,10 @@ class Attribute extends Entity {
 
     async beforeSaving() {
         if (!this.value) {
+            if (this.type === 'relation') {
+                throw new Error(`Cannot save relation ${this.name} since it does not target any note.`);
+            }
+
             // null value isn't allowed
             this.value = "";
         }