How to sum fields value from Observable with Angular 4 Firestore array

Question

I'm new in Cloud FireStore and have been searching for many days without results, for a simple solution on how can I sum some fields value of an array coming from Observable Angular Firestore. I have a list of rows and I want sum and have the column total, here is part of my code:

constructor(private afs: AngularFirestore, private router: Router,  private route: ActivatedRoute) { 
    this.negocio = this.route.snapshot.queryParams['negocioId']
    this.movtosCollectionRef = this.afs.collection('movtos', ref => ref.where('empresa', '==', this.negocio));


    this.movtos$ = this.movtosCollectionRef.snapshotChanges().map(actions => {
        return actions.map(action => {
          const data = action.payload.doc.data() as Movtos;
          const id = action.payload.doc.id;
          return { id, ...data };
        });


    });   
  }

In my template I make the browse :

      <tbody>
            <tr *ngFor="let item of movtos$ | async" [class.completed]="item.completed">
                <td>{{ item.indate | date: 'dd/MM/yyyy - HH:mm:ss'}}</td>
                <td>{{ item.concepto }} </td>
                <td>{{ item.tipomov }} </td>
                <td>{{ item.inuser }} </td>
                <td>{{ item.importe |  currency:'USD':true:'3.2-2' }}</td>
                <td>
                    <a class="btn btn-info" (click)="editaEmpresa(item.id)" tooltip="Edita Movimiento" placement="top"><span class="glyphicon glyphicon-edit" aria-hidden="true"></span></a>

                </td>

            </tr>   

        </tbody>

And I want to get the sum of item.importe field. Does any body know how to do this? Best regards, Caribesoft

Rahul Singh · Accepted Answer · 2017-12-24 18:34:58Z

1

You can use map on the Observable to return the sum. eg

getSum(): Observable<number> {
    return this.observable
       .map(arr => arr.reduce((a, b) => a + b.value, 0));
}

answered Dec 24, 2017 at 18:34

Rahul Singh

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5 Comments

CaribeSoft Over a year ago

Thanks Rahul for your response, but where do I get "this.observable" ? I tried with this.movtos$ but it does not work.

Rahul Singh Over a year ago

Are you getting the response as a promise? Then you can use Observed.fromPromise()

CaribeSoft Over a year ago

I'm trying this -> getSum(): Observable<number> { return this.movtos$ .map(arr => arr.reduce((a, b) => a + b.importe, 0)); } but I get TOTAL: [object Object]

Rahul Singh Over a year ago

Can you try and log what the object holds?

CaribeSoft Over a year ago

Hi, thank you, I resolved using Subscription this way : this.totalSubscription = this.movtos$.subscribe((list:any) => { this.total = 0; let fixedLength = list.length || 0; for (let i = 0; i < fixedLength; i++) { this.total += list[i].importe; } }

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