angular2 material - md-error and username does not exists from a server call - angular2-forms

Question

This is my login form. If a user puts in a wrong username and does not exists on the server then how do I use md-error to display message?
E.g.
Here is the json I get back for auth from a submit:
{is_username: false, status: false}
Here is how I set up my form:
this.myForm = fb.group({
username: ['',[Validators.required]],
password: ['',[Validators.required]]
})
Here is my HTML:
<md-input-container>
<input mdInput type="email" placeholder="Username" [(ngModel)]="username" formControlName="username" style="width: 300px;outline: none;">
<md-error *ngIf="myForm.get('username').hasError('required')">Username is required</md-error>
<md-error *ngIf="myForm.get('username').hasError('username_exists')">Username does exist</md-error>
</md-input-container>
So..how do I trigger a md-error message if is_username is false?

Answer 1

I've been doing server side validation like this.
<md-input-container>
<input mdInput type="email" placeholder="Username" [(ngModel)]="username" formControlName="username" style="width: 300px;outline: none;">
<md-error *ngIf="myForm.get('username').hasError('required')">Username is required</md-error>
<md-error *ngIf="myForm.controls['username'].hasError('username_exists')">{{myForm.controls['username'].errors['username_exists']}}</md-error>
</md-input-container>
Then, set the error after you get the response from the server.
https://angular.io/api/forms/AbstractControl#setErrors
this.myForm.controls['username'].setErrors({
'username_exists': 'Username does exist'
});
I typically return validation error messages from the server as well, which could be used here.