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
  const all = await cds.transaction(req).run(() =>
    OrderItems.map(order =>
      UPDATE(Books)
        .set('stock -=', order.amount)
        .where('ID =', order.book_ID)
        .and('stock >=', order.amount)
    )
  )
  all.forEach((affectedRows, i) => {
    if (affectedRows === 0) req.error(409, `${OrderItems[i].amount} exceeds stock for book #${OrderItems[i].book_ID}`)
  })
}