demo -> done

done/server.js

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+const cds = require('@sap/cds')
+module.exports = cds.server
+
+
+cds.once('bootstrap',(app)=>{
+  app.use('/vue',_from('@capire/bookshop/app/vue'))
+})
+
+
+
+cds.once('served', async ()=>{
+
+  // Connect to services we want to mashup below...
+  const S4bupa = await cds.connect.to('API_BUSINESS_PARTNER')   //> external S4 service
+  const admin = await cds.connect.to('AdminService')            //> local domain service
+  const db = await cds.connect.to('db')                         //> our primary database
+
+  // Reflect CDS definition of the Suppliers entity
+  const Suppliers = S4bupa.entities
+
+  // admin.prepend (()=>{
+
+    // Delegate Value Help reads for Suppliers to S4 backend
+    admin.on ('READ', 'Suppliers', req => {
+      console.log ('>> delegating to S4 service...')
+      return S4bupa.read(Suppliers) .where (req.query.where)
+    })
+
+
+    // Replicate Supplier data when Books are edited
+    admin.on (['CREATE','UPDATE'], 'Books', async req => {
+      const { supplier } = req.data
+      if (supplier) {
+        let cached = await db.read (Suppliers, supplier)
+        if (!cached) await replicate (supplier,'initial')
+      }
+    })
+
+  // })
+
+  // Subscribe to changes in the S4 origin of Suppliers data
+  S4bupa.on ('BusinessPartner/Changed', async msg => {
+    const ID = msg.businessPartner.KEYS
+    const cached = await db.read (Suppliers, sup => sup.ID) .where ({ ID })
+    for (let each of cached) replicate (each, 'update')
+  })
+
+  // Helper function to replicate Suppliers data
+  async function replicate (ID,_initial) {
+    let data = await S4bupa.read (Suppliers, ID)
+    if (_initial) return db.insert (data) .into (Suppliers)
+    else return db.update (Suppliers,ID) .with (data)
+  }
+
+})
+
+
+
+
+
+
+
+// -----------------------------------------------------------------------
+// Helper for serving static content from npm-installed packages
+const {static} = require('express')
+const {dirname} = require('path')
+const _from = target => static (dirname (require.resolve(`${target}/index.html`)))