默认情况下，使用无功形式的3下拉菜单（角度6），它将显示数据对下拉菜单值的依赖性

Question

在角度6项目中，我有3个下拉菜单，彼此依赖。默认情况下，必须选择这些值并提交，然后它将显示数据。

我有下拉菜单的默认值。单击提交按钮，它将显示数据，更改下拉值后，提交也将起作用。

<form [formGroup] = "addTestFilter" (ngSubmit)="getTestsBysubject()">
   <select id="selectBoard" formControlName="board" name="selectBoard" class="form-control" 
     (change) = "getGradeByBoard(selectBoard)" required [(ngModel)]="selectBoard">
      <option *ngFor="let board of boards" [value]="board._id">
         {{board.boardName}}
      </option>                          
   </select>
  <select id="selectGrade" formControlName="grade" name="selectGrade" class="form-control" 
     (change) = "getGradeBySubject(selectGrade)" required [(ngModel)]="selectGrade">
     <option *ngFor="let grade of grades" [value]="grade._id">
        {{grade.grade}}
     </option>                          
 </select>
 <select id="select3" formControlName="subject" name="selectSubject" class="form-control" 
      required [(ngModel)]="selectSbject">
     <option *ngFor="let sub of subjects" [value]="sub._id">
        {{sub.subjectName}}
     </option>                          
 </select>
 <button type="submit" class="btn btn-success mr-1 fa fa-search"></button>
</form>

默认情况下，我必须不单击提交按钮来获取数据