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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`)
+  })
+}