cloud-cap-samples/bookshop/srv/cat-service.js

const cds = require('@sap/cds')
module.exports = async function (){

  const db = await cds.connect.to('db') // connect to database service
  const { Books } = db.entities         // get reflected definitions

  // Reduce stock of ordered books if available stock suffices
  this.on ('submitOrder', async req => {
    const {book,amount} = req.data
    const n = await UPDATE (Books, book)
      .with ({ stock: {'-=': amount }})
      .where ({ stock: {'>=': amount }})
    n > 0 || req.error (409,`${amount} exceeds stock for book #${book}`)
  })

  // Add some discount for overstocked books
  this.after ('READ','Books', each => {
    if (each.stock > 111) {
      each.title += ` -- 11% discount!`
    }
  })
}