article: Article;
mainGroups: Group[];
subGroups: Group[];

ngOnInit() {
    this._groupService.getAll().subscribe(groups => this.mainGroups = groups);
    this._groupService.getAllSubGroups().subscribe(subgroups => this.subGroups = subgroups);
  }

<div class="form-group">
  <label class="control-label dash-control-label col-xs-3">Main group:</label>
  <div class="col-xs-9">
    <select class="form-control dash-form-control select2" style="width: 100%;"
            data-minimum-results-for-search="Infinity" name="articleGroups" required [(ngModel)]="article.mainGroup">
      <option disabled [ngValue]="null">-/-</option>
      <option [ngValue]="group" *ngFor="let group of mainGroups">{{group.title}}</option>
    </select>
  </div>
</div>

<!--Sub group-->
<div class="form-group">
  <label class="control-label dash-control-label col-xs-3">Sub group:</label>
  <div class="col-xs-9">
    <select class="form-control dash-form-control select2" style="width: 100%;" name="subGroup" required [(ngModel)]="article.subGroup">
      <option disabled [ngValue]="null">-/-</option>
      <option [ngValue]="subgroup" *ngFor="let subgroup of subGroups">{{subgroup.title}}</option>
    </select>
  </div>
</div>

<select class="form-control" [(ngModel)]="size" name="sizeValue" #sizeValue="ngModel">
  <option *ngFor="let size of sizes" [value]="size">{{size}}</option>
</select>