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graphql/srv/admin-service.cds

@@ -1,5 +0,0 @@
-using {sap.capire.graphql} from '../db/schema';
-
-extend service AdminService with {
-  entity Chapters as projection on graphql.Chapters;
-}

graphql/srv/bookshop-service.cds

@@ -0,0 +1,11 @@
+using {
+  sap.capire.bookshop,
+  sap.capire.graphql
+} from '../db/schema';
+
+service BookshopService {
+  entity Books    as projection on bookshop.Books;
+  entity Authors  as projection on bookshop.Authors;
+  entity Chapters as projection on graphql.Chapters;
+  entity Orders   as projection on graphql.Orders;
+}

graphql/srv/bookshop-service.js

@@ -0,0 +1,11 @@
+module.exports = function() {
+  const { Orders, Books } = this.entities
+
+  this.before('CREATE', Orders, async function(req) {
+    const { book_ID, quantity } = req.data
+
+    // reduce the stock, if enough are available, else reject the order
+    const applied = await UPDATE(Books, book_ID).set({ stock: { '-=': quantity } }).where({ stock: { '>=': quantity }})
+    if (!applied) req.reject(400, `Sorry, ${quantity} are not in stock`)
+  })
+}