Default Symfony User Entity with Form Class

I wish to create A Form class for my Login form that uses the Default Symfony User class.

I followed the book here: http://symfony.com/doc/current/book/security.html#using-a-traditional-login-form

Now I wish to turn that html into a Form class for which I did:

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class LoginForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('_username', 'text');
        $builder->add('_password', 'password');
    }
    
    public function getName()
    {
        return 'login';
    }
    
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
                'data_class' => 'Symfony\Component\Security\Core\User\User',
        ));
    }
}

this should work.

Now I try to call it like this:

$user = new \Symfony\Component\Security\Core\User\User('ana', 'anon');
$form = $this->createForm(new LoginForm(), $user);

And I got 2 problems:

1. the user class requires that I specify an username and password which is not what I want at all. I want this to be a login form, not create a user form.

2. it throws an error when I try to render:

   Neither property "_username" nor method "get_username()" nor method 
   "is_username()" exists in class "Symfony\Component\Security\Core\User\User"
   500 Internal Server Error - InvalidPropertyException
   

What am I doing wrong? And I don't want to use FOSUserBundle yet, I wish to learn the default Symfony classes, you can call me an idiot for this.

EDIT: I removed the the underscores(_) from the field names and it started to work but the first problem still stands, and now I got a a new problem:

If I try to submit my form all I get back is Bad credentials credentials.

login.html.twig:

<form action="{{ path('main_login_check') }}" method="post">
    {{ form_widget(form) }}

    <button type="submit">login</button>
</form>

this is the Action that calls it:

public function loginAction()
    {
        $request = $this->getRequest();
        $session = $request->getSession();
        $user = new \Symfony\Component\Security\Core\User\User('ana', 'anon');
        $form = $this->createForm(new LoginForm(), $user);
        // get the login error if there is one
        if ($request->attributes->has(SecurityContext::AUTHENTICATION_ERROR))
        {
            $error = $request->attributes->get(
                    SecurityContext::AUTHENTICATION_ERROR
                    );
        }
        else 
        {
            $error = $session->get(SecurityContext::AUTHENTICATION_ERROR);
            $session->remove(SecurityContext::AUTHENTICATION_ERROR);
        }
        
        return $this->render(
                'BattlemamonoMainBundle:Security:login.html.twig',
                array(
                        // last username entered by the user
                        'last_username'   => $session->get(SecurityContext::LAST_USERNAME),
                        'error'           => $error,
                        'form'            => $form->createView(),
                        )
                );
    }