Flask wtforms check if username already exists / flashing errors

from flask_wtf import FlaskForm
from wtforms.validators import DataRequired
from wtforms.validators import ValidationError

class RegisterForm(FlaskForm):
    
    # ...
    
    username = StringField(
        'Username',
        validators=[
            DataRequired(),
            # ...
        ])

    # ...
    
    def validate_username(self, field):
        if User.query.filter_by(username=field.data).first():
            raise ValidationError('Username already in use.')