django在更新保存时删除图像

我有以下清理方法,如果图像太大,可以通过该方法缩小图像:

类别CompanyForm(forms.ModelForm): 类元: 型号=公司 排除=(“有限”)

def clean_image(self):
    image_field = self.cleaned_data.get('image')

    if image_field:
        reduced_image = reduce_image(image_field, 550)
        return reduced_image
    else:
        raise forms.ValidationError('You must upload a square logo image')

    return image_field

我的reduce图像如下所示:

def reduce_image(image_field, height_max):

    if image_field:

        try:
            image_file2 = BytesIO(image_field.read())
            image = Image.open(image_file2).convert('RGB')
            print(image)
        except:
            #raise ValidationError('There was a problem with the image, please try another.')
            print('returning from image errror')
            return image_field

        w, h = image.size
        print('image width:'+str(w))
        print('image height:'+str(h))

        if h > height_max:
            print('height toolarge')
            ratio = h/float(w)
            new_height = ratio * height_max

            image = image.resize((int(height_max), int(new_height)), Image.ANTIALIAS)
            image_file = BytesIO()
            image.save(image_file, 'JPEG', quality=90)
            image_field.file = image_file
            print(image_file)


    return image_field

我第一次保存时,就没有问题了。当我保存第二次更新模型时,它会删除图像。

为什么会发生这种情况?