问未捕获的列: SQLSTATE[23000]：完整性约束冲突: 1048列不能为空

EN

Uncaught PDOException: SQLSTATE[23000]: Integrity constraint violation: 
1048 Column 'created_at' cannot be null

Column  Type    Comment
id  int(11) Auto Increment   
name    varchar(255)     
created_at  datetime     
last_login  datetime NULL   

<?php
// create_user.php
use Doctrine\ORM\Mapping as ORM;
require_once "bootstrap.php";
require 'vendor/autoload.php';
use Db\User;

$newUsername = $argv[1];

$user = new User();
$user->setName($newUsername);

$entityManager->persist($user);
$entityManager->flush();

echo "Created User with ID " . $user->getId() . "\n";

<?php
// src/User.php
namespace Db;
use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
* @ORM\Entity @ORM\Table(name="user")
**/
class User
{
/** @ORM\Id @ORM\Column(type="integer") @ORM\GeneratedValue **/
protected $id;

/** @ORM\Column(type="string") **/
protected $name;

/** @ORM\Column(type="datetime") **/
protected $created_at;

/** @ORM\Column(type="datetime", nullable=true) **/
protected $last_login;