title: Error trying to diff '[object Object]'. Only arrays and iterables are allowed 4 times
body: <p>I have 4 dropdowns 
   -FromYear-
   -FromMonth-
   -ToYear-
   -ToMonth-
  When I select an year from 1st and 3rd dropdown, it should get an array of months to populate my other dropdowns.
   I get my months with selected year's id using selectFromYearsEvent</p>
  
  <p>This is my Component </p>
  
  <pre><code>export class CalendarComponent implements OnInit {
  
    year: number[];
  
    selectYears: Year[];
  
    selectedFromYear: Year;
    selectedToYear: Year;
  
    selectFromMonths: Month[];
    selectToMonths: Month[];
  
    selectedFromMonth: Month;
    selectedToMonth: Month;
  
    selectedMonths: Month[];
  
    months: Month[];
  
    constructor(private calendarService: CalendarService, private monthsService: MonthsService) {
  
    }
  
    ngOnInit() {
          this.getYears();
        }
  
    getYears() {
      this.calendarService.getYears().subscribe(y =&gt; {
        this.selectYears = y;
      });
    }
  
    getMonth() {
      this.calendarService.getYear(this.selectedFromYear.id).subscribe(y =&gt; {
        this.selectedFromYear.months = y;
        this.selectFromMonths = y;
      });
    }
  
    selectFromYearsEvent(newYear) {
      this.selectedFromYear = newYear;
      this.getMonth();
  
    }
  }
  </code></pre>
  
  <p>and this is my HTML code</p>
  
  <pre><code>&lt;div class="container-calendar"&gt;
          &lt;div class="calendar-dates"&gt;
          &lt;!-- from part --&gt;
          &lt;div class="from-date"&gt;
            &lt;div class="from-date-dropdown"&gt;
              &lt;select name="fromYear" [(ngModel)]="selectYears" (ngModelChange)="selectFromYearsEvent($event)"&gt;      &lt;!-- [(ngModel)]="selectedFromYear"&gt; --&gt;
                &lt;option name="fromYear2" *ngFor="let year of selectYears" [ngValue]="year"&gt;                   
                    {{year.number}}
                &lt;/option&gt;
              &lt;/select&gt;
            &lt;/div&gt;
          &lt;/div&gt;
          &lt;div class="from-date"&gt;
            &lt;div class="state-dropdown"&gt;
              &lt;select name="fromMonth" [(ngModel)]="selectFromMonths" (ngModelChange)="selectFromMonthEvent($event)"&gt; &lt;!-- [(ngModel)]="selectedFromMonth"&gt; --&gt;
                &lt;option *ngFor='let month of selectFromMonth' [ngValue]="month"&gt; 
                    {{month.name}}
                &lt;/option&gt;
              &lt;/select&gt;
            &lt;/div&gt;
          &lt;/div&gt;
  </code></pre>
  
  <p></p>