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reviews/db/schema.cds

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+namespace sap.capire.reviews;
+using { User } from '@sap/cds/common';
+
+// Reviewed subjects can be any entity that is uniquely identified
+// by a single key element such as a UUID
+type ReviewedSubject : String(111);
+
+entity Reviews {
+  key ID   : UUID;
+  subject  : ReviewedSubject;
+  reviewer : User;
+  rating   : Rating;
+  title    : String(111);
+  text     : String(1111);
+  date     : DateTime;
+  likes    : Composition of many Likes on likes.review = $self;
+  liked    : Integer default 0; // counter for likes as helpful review (count of all _likes belonging to this review)
+}
+
+type Rating : Decimal(3,2) enum {
+  Best  = 5;
+  Good  = 4;
+  Avg   = 3;
+  Poor  = 2;
+  Worst = 1;
+}
+
+entity Likes {
+  key review : Association to Reviews;
+  key user   : User;
+}

reviews/index.cds

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+using from './srv/reviews-service';

reviews/package.json

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+{
+  "name": "@sap/capire-reviews",
+  "version": "1.0.0",
+  "description": "A reuse service providing generic means to add reviews and ratings to target objects, e.g. products.",
+  "repository": "https://github.com/SAP-samples/cloud-cap-samples.git",
+  "license": "SAP SAMPLE CODE LICENSE",
+  "dependencies": {
+    "@sap/cds": "latest",
+    "express": "*"
+  },
+  "scripts": {
+    "start": "cds run --in-memory?",
+    "watch": "cds watch"
+  },
+  "files": [
+    "db",
+    "srv",
+    "index.cds"
+  ],
+  "cds": {
+    "requires": {
+      "db": {
+        "kind": "sql"
+      },
+      "messaging": {
+        "kind": "file-based-messaging"
+      }
+    }
+  }
+}

reviews/srv/reviews-service.cds

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+using { sap.capire.reviews as my } from '../db/schema';
+namespace sap.capire.reviews;
+
+service ReviewsService {
+  // Sync API
+  entity Reviews as projection on my.Reviews excluding { likes }
+  action like (review:Reviews.ID);     // TODO: can be a bound action in OData
+  action unlike (review:Reviews.ID);   // TODO: can be a bound action in OData
+
+  // Async API
+  event reviewed : { subject: Reviews.subject; rating: Decimal(2,1) };
+
+  // Input validation
+  annotate Reviews with {
+    subject  @mandatory;
+    title    @mandatory;
+    rating   @mandatory @assert.enum;
+  }
+
+  // Auto-fill reviewers and review dates
+  annotate Reviews with {
+    reviewer @cds.on.insert:$user;
+    date     @cds.on.insert:$now;
+    date     @cds.on.update:$now;
+  }
+
+}
+
+
+// Access control restrictions
+annotate ReviewsService.Reviews with @restrict_:[
+  { grant:'READ',   to:'any' },                 // everybody can read reviews
+  { grant:'CREATE', to:'authenticated-user' },  // users must login to add reviews
+  { grant:'UPDATE', to:'authenticated-user', where:'reviewer=$user' },
+  { grant:'DELETE', to:'admin' },
+];
+
+annotate ReviewsService with @restrict_:[
+  { grant:'like', to:'identified-user' },
+  { grant:'unlike', to:'identified-user', where:'user=$user' },
+];

reviews/srv/reviews-service.js

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+const cds = require ('@sap/cds')
+module.exports = cds.service.impl (function(){
+
+  // Get the CSN definition for Reviews from the db schema for sub-sequent queries
+  // ( Note: we explicitly specify the namespace to support embedded reuse )
+  const { Reviews, Likes } = this.entities ('sap.capire.reviews')
+
+  // Emit an event to inform subscribers about new avg ratings for reviewed subjects
+  // ( Note: req.on.succeeded ensures we only do that if there's no error )
+  this.after (['CREATE','UPDATE','DELETE'], 'Reviews', async(_,req) => {
+    const {subject} = req.data
+    const {rating} = await cds.transaction(req) .run (
+      SELECT.one (['round(avg(rating),2) as rating']) .from (Reviews) .where ({subject})
+    )
+    req.on ('succeeded', ()=>{
+      console.log ('< emitting:', 'reviewed', { subject, rating })
+      this.emit ('reviewed', { subject, rating })
+    })
+  })
+
+  // Increment counter for reviews considered helpful
+  this.on ('like', (req) => {
+    if (!req.user)  return req.reject(400, 'You must be identified to like a review')
+    const {review} = req.data, {user} = req
+    const tx = cds.transaction(req)
+    return tx.run ([
+      INSERT.into (Likes) .entries ({review_ID: review, user: user.id}),
+      UPDATE (Reviews) .set({liked: {'+=': 1}}) .where({ID:review})
+    ]).catch(() => req.reject(400, 'You already liked that review'))
+  })
+
+  // Delete a former like by the same user
+  this.on ('unlike', async (req) => {
+    if (!req.user)  return req.reject(400, 'You must be identified to remove a former like of yours')
+    const {review} = req.data, {user} = req
+    const tx = cds.transaction(req)
+    const affectedRows = await tx.run (DELETE.from (Likes) .where ({review_ID: review,user: user.id}))
+    if (affectedRows === 1)  return tx.run (UPDATE (Reviews) .set ({liked: {'-=': 1}}) .where ({ID:review}))
+  })
+
+})

reviews/tests/messaging.test.js

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+const _model = __dirname+'/..'
+const cds = require ('@sap/cds')
+
+describe('messaging tests', ()=>{
+
+    it ('should bootstrap sqlite in-memory db', async()=>{
+        const db = await cds.deploy (_model) .to ('sqlite::memory:')
+        expect (db.model) .toBeDefined()
+    })
+
+    let srv
+    it ('should serve reviews services', async()=>{
+        srv = await cds.serve('ReviewsService') .from (_model)
+        expect (srv.name) .toMatch ('ReviewsService')
+    })
+
+    let N=0, received=[], M=0
+    it ('should add messaging event handlers', ()=>{
+        srv.on('reviewed', (msg)=> received.push(msg))
+    })
+
+    it ('should add more messaging event handlers', ()=>{
+        srv.on('reviewed', ()=> ++M)
+    })
+
+    it ('should add review', async ()=>{
+        const review = {
+            ID: 111 + (++N), // FIXME: why does the generic handler not fill this in automatically ?!? --> it does so when the request comes in via Postman / OData
+            subject: "201", title: "Captivating", rating: N
+        }
+        const response = await srv.create ('Reviews') .entries (review)
+        expect (response) .toMatchObject (review)
+    },100)
+
+    it ('should add more reviews', ()=> Promise.all ([
+        // REVISIT: mass operation should trigger one message per entry
+        // srv.create('Reviews').entries(
+        //     { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N },
+        //     { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N },
+        //     { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N },
+        //     { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N },
+        // ),
+        srv.create ('Reviews') .entries (
+            { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N }
+        ),
+        srv.create ('Reviews') .entries (
+            { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N }
+        ),
+        srv.create ('Reviews') .entries (
+            { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N }
+        ),
+        srv.create ('Reviews') .entries (
+            { ID: 111 + (++N),  subject: "201", title: "Captivating", rating: N }
+        ),
+    ]) ,100)
+
+    it ('should have received all messages', async()=> {
+        await new Promise((done)=>setImmediate(done))
+        expect(M).toBe(N)
+        expect(received.length).toBe(N)
+        expect(received.map(m=>m.data)).toEqual([
+            { subject: '201', rating: 1 },
+            { subject: '201', rating: 1.5 },
+            { subject: '201', rating: 2 },
+            { subject: '201', rating: 2.5 },
+            { subject: '201', rating: 3 },
+        ])
+    })
+})