class M(models.Model):
    title = models.CharField(max_length=50, verbose_name='主题')

    c_user = models.CharField(verbose_name='创建人', max_length=256, null=True, blank=True)
    u_user = models.CharField(verbose_name='更新人', max_length=256, null=True, blank=True)

    def save(self, *args, **kwargs):
        if not self.pk:
            self.c_user = self.user
        else:
            self.u_user = self.user
        super().save(*args, **kwargs)