Django beginner. When I rewritten the model class, the following error appeared.
The reason for this is that I don't think that the attribute has been given to a new post, but I want to know how to solve it.

Problem code
class Post (models.Model):
    author = models.ForeignKey (settings.AUTH_USER_MODEL, on_delete = models.CASCADE)
    title = models.CharField (max_length = 200)
    text = models.TextField ()
    created_date = models.DateTimeField (default = timezone.now ())
    published_date = models.DateTimeField (blank = True, null = True)
    def publish (self):
        self.published_date = timezone.now ()
        self.save ()
    def __str __ (self):
        return self.title


OperationalError at/admin/blog/post/4/change/ no such column: blog_post.published_date I got an error with