Override attempt auth method in laravel 5

I'm having a big problem with Auth:attempt method in laravel 5.2, and I wish I can override it.
I have the class users, by default, but doesn't containt the attribut 'email', this last one is containing in an other class user_contacts, with relation one to one with users.
Can I override this function? because by default, it takes 'email', which is not in users in my case I tried to make that in the model, but not working:

lass User extends Authenticatable

{

public function __construct(array $attr = array())
{
    parent::__construct($attr);

    $this->email = $this->user_contacts()->email;
}

/**
 * The attributes that are mass assignable.
 *
 * @var array
 */
protected $fillable = [
    'name', 'email', 'password',
];

/**
 * The attributes that should be hidden for arrays.
 *
 * @var array
 */
protected $hidden = [
    'password', 'remember_token',
];

public function user_contacts()
{
    return $this->hasOne('App\UserContact', 'user_id', 'id');
}

}

It just says that the email field is not defined.
Any suggestion plz?
Thanks alot.

via Chebli Mohamed