docs: tests + docs

packages/medusa-core-utils/src/transform-idable-fields.js

@@ -1,8 +1,18 @@
+/**
+ * Takes an object and a list of fields to tranform in that object. If the field
+ * exists on the object and its value is a string it will append `_id` to the 
+ * field name. This is used when allowing API calls to hold either ID or object
+ * values in the payload. The method returns a new object with the 
+ * transformation.
+ * @param {Object} obj - the object to transform
+ * @param {Array<String>} fields - the fields to apply transformation to
+ * @returns {Object} the transformed object
+ */
 export const transformIdableFields = (obj, fields) => {
   const ret = { ...obj }
 
   for (const key of fields) {
-    if (key in obj && typeof key === "string") {
+    if (key in obj && typeof ret[key] === "string") {
       ret[`${key}_id`] = ret[key]
       delete ret[key]
     }