Django图片字段设置：防默认图误删、清旧文件及排障

背景与初始实现 #

尽管多数现代网站采用OSS存储图片，但仍希望搭建后端本地管理缩略图。Django的ImageField使用存在难点，修改图片引用涉及models.py、views.py、forms.py三处。

此前直接通过request.user.profile.image = self.files['image']设置图片，模型中为ImageField指定了默认值：

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    image = ProcessedImageField(
        default='profile/default.jpg', 
        upload_to='profile', 
        processors=[ResizeToFill(*THUMBNAIL_SIZE)],
        format='JPEG',
        options={'quality': THUMBNAIL_QUALITY}, 
    )

初始实现的问题 #

测试后发现两个问题：

• 默认图片被误删
• 重置非默认图片时，旧文件未删除，占用存储空间

自定义全局函数及异常 #

为此编写全局setImage函数，意图统一处理图片设置：

from django.conf import settings

def setImage(instance, attr, file):
    """ instance will be saved """
    if file:
        ifield = getattr(instance, attr)
        # old reference, can be default
        iurl = ifield.url
        # default
        durl = settings.MEDIA_URL + instance._meta.get_field(attr).get_default()
        if iurl != durl:
            # old reference is not default
            # delete old reference and free up space
            ifield.delete(save=True)
        # set new file 
        setattr(ifield, attr, file)
    instance.save()

但测试发现图片始终无法设置，已排除表单enctype、Ajax参数、模型save方法重写等问题。日志显示新文件已传入，但保存后仍为默认图：

======
Screen Shot 2022-11-03 at 10.59.41 pm.png
/media/profile/default.jpg
/media/profile/default.jpg
----res
Screen Shot 2022-11-03 at 10.59.41 pm.png
Screen Shot 2022-11-03 at 10.59.41 pm.png
--- ins
tracey
--- after save
/media/profile/default.jpg
profile/default.jpg

请问为何图片无法设置？

修正setImage函数 #

经测试发现仅一行变量错误，修正后的setImage函数：

def setImage(instance, attr, file):
    """ instance will be saved """
    if file:
        ifield = getattr(instance, attr)
        iurl = ifield.url
        durl = settings.MEDIA_URL + instance._meta.get_field(attr).get_default()
        if iurl != durl:
            ifield.delete(save=True)
        setattr(instance, attr, file)
        instance.save()

错误说明：原代码中setattr(ifield, attr, file)是给ImageField对象本身设置属性，而非给模型实例设置属性，导致新文件无法被正确关联到模型实例上，修正为直接给instance设置对应属性即可。

表单更新验证的clean_image方法 #

针对表单更新验证，提供可用的clean_image方法：

def clean_image(form, attr:str, field:str):
    """ 
    form.instance.attr -> ImageFile
    form.cleaned_data[field] -> ImageFile
    """
    upload_img = form.cleaned_data[field]
    if form.instance:
        # condition: update
        # create does not matter 
        ifield = getattr(form.instance, attr)
        iurl = ifield.url
        if isinstance(upload_img, InMemoryUploadedFile):
            # upload new file
            # else, the file is not changed
            durl = settings.MEDIA_URL + form.instance._meta.get_field(attr).get_default()
            if iurl != durl:
                ifield.delete(save=True)
    return upload_img

使用示例 #

class Project(models.Model):
    image = ProcessedImageField(
        default='projects/default.jpg', 
        upload_to='projects', 
        processors=[ResizeToFill(*THUMBNAIL_SIZE)],
        options={'quality': THUMBNAIL_QUALITY}, 
        format='JPEG',
    )

class ProjectCreateForm(forms.ModelForm):
    class Meta:
        model = Project
        fields = ['name', 'image']
    
    def clean_image(self):
        return clean_image(self, 'image', 'image')

内容的提问来源于stack exchange，提问作者Weilory