cat-service.js

const cds = require('@sap/cds')

class CatalogService extends cds.ApplicationService {
  init() {
    const { Books } = cds.entities('sap.capire.bookshop')
    const { ListOfBooks } = this.entities

    // 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 }
    })

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

    return super.init()
  }
}

module.exports = { CatalogService }