模型表单中的排除字段仍为必填字段

class PostForm(forms.ModelForm):
    post_type = forms.ChoiceField(widget=forms.RadioSelect(attrs={'name': 'radioInline'}), choices=POST_CHOICES)


    class Meta:
        model = Post
        fields = ('title','desc','image','url',) 

我有以下型号:

@python_2_unicode_compatible
class Post(models.Model):  
    entity = models.ForeignKey('companies.Entity')
    title = models.CharField('Post Title', max_length=128, unique=True) 
    desc = models.TextField('Description', blank=True, null=True)
    post_type = models.IntegerField(choices=POST_CHOICES)
    image = models.ImageField('Post Image', upload_to='post', blank=True, null=True)
    url = models.URLField(max_length=255, blank=True, null=True)
    slug = models.SlugField(blank=True, null=True, unique=True)
    created_at = models.DateTimeField(auto_now_add = True)
    updated_at = models.DateTimeField(auto_now = True)

当我提交表单时,我得到了错误:

我想在表单后填充此字段。是有效方法。

既然这个字段不在必填字段元组中,那么它不应该是必填字段吗?

我还尝试添加:

post_type = models.IntegerField(choices=POST_CHOICES, blank=True)

虽然我也犯了同样的错误。