angular 9 ui have a dropdown state and city defualt value state id=-1 and city=-1 ,on buton click ngform validation verify f.valid and city ='-1' then display erors and stateid='-1' display erors
[ngClass]="{'is-valid': f.submitted && cityID!=='-1', 'is-invalid' : f.submitted && reportingRangeTypeID=='-1'}"
(ngSubmit)="f.form.valid ? ButtonClick() :
(!cityID.valid && showErrorAlert('Password is required', 'Please enter a valid password'))">
<select *ngIf="model" [(ngModel)]="model.cityid" required class="form-control" #cityid="ngModel" id="cityID" name="rcityID"
[ngClass]="{'is-valid': f.submitted && cityID==='-1', 'is-invalid' : f.submitted && !city=='-1'}"
">
<option *ngFor="let a of model.cities" [ngValue]="a.value">{{ a.text}} </option>
</select>
onbuton click form is true even cityid is -1.