Build a social Network with flask

from flask_wtf import Form
from wtforms import StringField, PasswordField
from wtforms.validators import (DataRequired, Regexp, ValidationError, Email,
                                Length, EqualTo)

from models import User


def email_exists(form, field):
    if User.select.where(User.email == field.data).exists():
        raise ValidationError('User with that email already exists.')

def name_exists(form, field):
    if User.select.where(User.username == field.data).exists():
        raise ValidationError('User with that name already exists.')

class RegisterForm(Form):
    username = StringField(
        'Username',
        validators=[
            DataRequired(),
            Regexp(
                r'^[a-zA-Z0-9_]+$',
                message=("Username should be one word, letters, "
                        "numbers and underscores only.")
            ),
            name_exists
        ])
    email = StringField(
        'Email',
        validators=[
            DataRequired(),
            Email(),
            email_exists
        ])
    password = PasswordField(
        'Password',
        validators=[
            DataRequired(),
            Length(min=2),
            EqualTo('password2', message='Password must match')
        ])
    password2 = PasswordField(
        'Confirm password',
        validators=[ DataRequired()]
    )