getInvoices() {
this.invCollection = this.afs.collection('invoices');
this.invoices = this.invCollection.snapshotChanges().pipe(
  shareReplay(1),
  map(actions => {
  return actions.map(action => ({
    key: action.payload.doc.id,
    ...action.payload.doc.data()
  }))
})
)

this.invList = this.invoices.source;

<mat-form-field class="ffield">
    <mat-select (selectionChange)="getInv($event)" placeholder="Choose invoice">
      <mat-option *ngFor="let invoice of invoices | async; trackBy: trackByFn" [value]="invoice.key">{{invoice.invNo}}</mat-option>
    </mat-select>
  </mat-form-field>


getInv(e) {
  this.sID = e.value; //id of selected invoice

  //TODO filter this.invList observable   }

this.invList.pipe(map(aaa => {return aaa.map(bbb => ())}), filter(flt => flt.key === this.sID)).subscribe(sss => (console.log(sss.invNo)))