Adopt repository structure

srv/cat-service.js

@@ -0,0 +1,26 @@
+const cds = require('@sap/cds')
+const { Books } = cds.entities
+
+/** Service implementation for CatalogService */
+module.exports = cds.service.impl(function() {
+  this.after ('READ', 'Books', each => each.stock > 111 && _addDiscount2(each,11))
+  this.before ('CREATE', 'Orders', _reduceStock)
+})
+
+/** Add some discount for overstocked books */
+function _addDiscount2 (each,discount) {
+  each.title += ` -- ${discount}% discount!`
+}
+
+/** Reduce stock of ordered books if available stock suffices */
+async function _reduceStock (req) {
+  const { Items: OrderItems } = req.data
+  return cds.transaction(req) .run (()=> OrderItems.map (order =>
+    UPDATE (Books) .set ('stock -=', order.amount)
+    .where ('ID =', order.book_ID) .and ('stock >=', order.amount)
+  )) .then (all => all.forEach ((affectedRows,i) => {
+    if (affectedRows === 0)  req.error (409,
+      `${OrderItems[i].amount} exceeds stock for book #${OrderItems[i].book_ID}`
+    )
+  }))
+}