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

// We are mashing up three services...
const admin = cds.connect.to('AdminService')
const bupa = cds.connect.to('API_BUSINESS_PARTNER')
const db = cds.connect.to('db')

// Using reflected definitions from connected services/database
const { Addresses: externalAddresses } = bupa.entities // projection on external addresses
const { Books, Addresses } = db.entities // entities in local database

console.log(Addresses._service.name, Addresses._service)

module.exports = admin => {
  // Handler to delegate ValueHelp requests to S/4 backend, fetching current user's addresses from there
  // admin.on ('READ', 'usersAddresses', (req) => {  // REVISIT: all requests go to auto-exposed Addresses
  admin.on('READ', 'Addresses', req => {
    return bupa.tx(req).run(
      SELECT.from(externalAddresses).where({
        contact: req.user.id || 'anonymous'
      })
    )
    // return bupa.tx(req) .read (externalAddresses) .where ({ contact: req.user.id || 'anonymous' }) //> FIXME: doesn't work !?!?
    // const { SELECT } = cds.ql(req) //> convenient alternative to bupa.transaction(req).run(SELECT...)
    // return SELECT.from (externalAddresses) .where ({ contact: req.user.id || 'anonymous' })
    //> this is applying projection from CDS model generically, i.e. the equivalent of:
    // const { A_BusinessPartnerAddress } = bupa.entities
    // return SELECT.from (A_BusinessPartnerAddress, a => {
    //   a.AddressID.as('ID'),
    //   a.BusinessPartner,
    //   a.Country.as('country'),
    //   a.CityName.as('cityName'),
    //   a.PostalCode.as('postalCode'),
    //   a.StreetName.as('streetName'),
    //   a.HouseNumber.as('houseNumber')
    // }) .where ({ BusinessPartner: req.user.id })
  })
}

// Replicate chosen addresses from S/4 when filing orders.
admin.before('PATCH', 'Orders', async req => {
  // REVISIT: Investigate strange behavior when first assigning A then B ... together with Fiori colleagues
  // const assigned = { ID: req.data.shippingAddress_ID, contact: req.data.shippingAddress_contact }
  const assigned = { ID: req.data.shippingAddress_ID, contact: req.user.id }
  if (!assigned.ID) return //> something else
  // const { SELECT, INSERT, UPSERT } = cds.ql(req) //> convenient alternative to <srv>.transaction(req).run(SELECT...)
  const local = db.transaction(req)
  const [replica] = await local.read(Addresses).where(assigned)
  if (replica) return //> already replicated
  // const [address] = await bupa.read (externalAddresses) .where ({  //> FIXME -> das erzeugt ein super-komisches Verhalten, mit zweimal PATCH, etc. !!!
  // const [address] = await bupa.tx(req) .read (externalAddresses) .where ({  //> FIXME !!!
  const [address] = await bupa
    .tx(req)
    .run(SELECT.from(externalAddresses).where(assigned))
  if (address) return local.create(Addresses).entries(address)
  // const address = await SELECT.one.from (externalAddresses) .where ({ //> TODO
  // FIXME: Zweimal INSERT -> constraint violation !!
  // if (address) return db.tx(req).run (INSERT.into (Addresses) .entries (address))
  // if (address) return UPSERT (Addresses) .entries (address) //> TODO
})

// Update local replicas when sources change in S/4.
bupa.on('BusinessPartner/Changed', async msg => {
  console.log('>> received:', msg.data)

  const BPID = msg.data.KEY[0].BUSINESSPARTNER // TODO: .KEY[0] >> revisit w/ Oliver
  const { SELECT, UPDATE } = cds.ql(msg) //> convenient alternative to <srv>.transaction(req).run(SELECT...)

  // fetch affected entries from local replicas
  const replicas = await SELECT.from(Addresses).where({ contact: BPID })
  if (replicas.length === 0) return //> not affected

  // fetch changed data from S/4 -> might be less than local due to deletes
  const changed = await SELECT.from(externalAddresses).where({
    contact: BPID,
    ID: replicas.map(a => a.ID) // where in
  })

  // update local replicas with changes from S/4
  const local = db.transaction(msg) //> using that variant to benefit from bulk runs
  return local.run(changed.map(a => UPDATE(Addresses, a.ID).with(a)))
})

// Validate incoming orders and reduce books' stocks.
admin.before('CREATE', 'Orders', async req => {
  const { Items } = req.data

  // validate input...
  if (!Items || Items.length === 0)
    return req.reject('Please order at least one item.')
  if (!req.data.shippingAddress_ID)
    return req.reject(
      'Please enter a valid shipping address.',
      'shippingAddress_ID'
    )
  // TODO: future way of doing that:
  // const {assert} = req
  // assert ('Items') .check (items => items && items.length, 'Please enter at least one order item')
  // assert ('shippingAddress') .mandatory() .exists()
  // if (req.hasErrors)  return

  // reduce stock on ordered books...
  const all = await db.tx(req).run(
    Items.map(each =>
      UPDATE(Books)
        .where('ID =', each.book_ID)
        .and('stock >=', each.amount)
        .set('stock -=', each.amount)
    )
  )
  all.forEach(
    (affectedRows, i) =>
      affectedRows > 0 ||
      req.error(
        409,
        `${Items[i].amount} exceeds stock for book #${Items[i].book_ID}`
      )
  )
})