Django，不能分配无，不允许空值

Question

我有这个models.py

import datetime
from django.db import models
from tinymce import models as tinymce_models
from filebrowser.fields import FileBrowseField

class ItemWithMedia(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

class Actual(ItemWithMedia):
    published = models.DateField('Published')
    title_hr = models.CharField('(hr)', max_length=200)
    title_en = models.CharField('(en)', max_length=200)
    body_text_hr = models.TextField('(hr)')
    body_text_en = models.TextField('(en)')

    def __unicode__(self):
        return self.title_hr

    class Meta:
        verbose_name = "Aktualno"
        verbose_name_plural = "Aktualni"
        ordering = ['-published']

当我尝试在管理站点中创建新项目时出现此错误:
不能分配无:“实际.published”不允许空值。

可能是什么问题呢？

Answer 1

 #for sql 'now()' value use
 published = models.DateField('Published', auto_now_add=True)
 #to allow sql null
 published = models.DateField('Published', null=True, blank=True)