this.datesIdCollection = this.afs.collection('dates');
this.afs.collection('dates').valueChanges().subscribe(dateIdAmounts =>{
  this.dateIdAmounts = dateIdAmounts as DateAmountId[];
});

updateOrAddToDates(){
    for(let dateAmount of this.dateIdAmounts){
      for (let selectedDate of this.selectedDates) {
        if(selectedDate.getTime()=== dateAmount.date.getTime()){
          this.datesCollection.doc(dateAmount.id).update(new DateAmount(selectedDate, this.amount));
        }else{
          this.pushToDates(new DateAmount(selectedDate, this.amount));
          break;
        }
      }
    }
  }

  pushToDates(datAmount){
    const datum = new Date();
    this.datesCollection.add({
      amount: datAmount.amount,
      date: datAmount.date
    }).then(
      (docRef) =>{
        this.datesCollection.doc(docRef.id).update({
          id: docRef.id
        })
      }
    );
  }