diff --git a/bookshop/srv/cat-service.js b/bookshop/srv/cat-service.js
index 3c557078..86352e73 100644
--- a/bookshop/srv/cat-service.js
+++ b/bookshop/srv/cat-service.js
@@ -1,29 +1,34 @@
-const cds = require('@sap/cds')
+module.exports = class CatalogService extends cds.ApplicationService { init() {
 
-class CatalogService extends cds.ApplicationService { init(){
-
-  const { Books } = cds.entities ('sap.capire.bookshop')
+  const { Books } = cds.entities('sap.capire.bookshop')
   const { ListOfBooks } = this.entities
 
+  // Add some discount for overstocked books
+  this.after('READ', ListOfBooks, each => {
+    if (each.stock > 111) each.title += ` -- 11% discount!`
+  })
+
   // Reduce stock of ordered books if available stock suffices
-  this.on ('submitOrder', async req => {
-    const {book,quantity} = req.data
-    if (quantity < 1) return req.reject (400,`quantity has to be 1 or more`)
-    let b = await SELECT `stock` .from (Books,book)
-    if (!b) return req.error (404,`Book #${book} doesn't exist`)
-    let {stock} = b
-    if (quantity > stock) return req.reject (409,`${quantity} exceeds stock for book #${book}`)
-    await UPDATE (Books,book) .with ({ stock: stock -= quantity })
-    await this.emit ('OrderedBook', { book, quantity, buyer:req.user.id })
-    return { stock }
+  this.on('submitOrder', async req => {
+    let { book:id, quantity } = req.data
+    let book = await SELECT.from (Books, id, b => b.stock)
+
+    // Validate input data
+    if (!book) return req.error (404, `Book #${id} doesn't exist`)
+    if (quantity < 1) return req.error (400, `quantity has to be 1 or more`)
+    if (quantity > book.stock) return req.error (409, `${quantity} exceeds stock for book #${id}`)
+
+    // Reduce stock in database and return updated stock value
+    await UPDATE (Books, id) .with ({ stock: book.stock -= quantity })
+    return book
   })
 
-  // Add some discount for overstocked books
-  this.after ('READ', ListOfBooks, each => {
-    if (each.stock > 111) each.title += ` -- 11% discount!`
+  // Emit event when an order has been submitted
+  this.after('submitOrder', async (_,req) => {
+    let { book, quantity } = req.data
+    await this.emit('OrderedBook', { book, quantity, buyer: req.user.id })
   })
 
+  // Delegate requests to the underlying generic service
   return super.init()
 }}
-
-module.exports = { CatalogService }