Django+imagekit上传图片时width字段违反非空约束的问题

使用版本：Django5.2.12，django-imagekit6.1.0

我的PostImage模型代码如下：

class PostImage(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE, null=True, related_name="images")
    image = ProcessedImageField(verbose_name=_("image"), storage=post_image_storage, upload_to=upload_to_post_image_directory, width_field="width", height_field="height", blank=False, null=True, format="JPEG", options={"quality": 100}, processors=[ResizeToFit(width=1024, upscale=False)])
    width = models.PositiveIntegerField(editable=False, null=False, blank=False)
    height = models.PositiveIntegerField(editable=False, null=False, blank=False)
    hash = models.CharField(_("hash"), max_length=64, blank=False, null=True)
    thumbnail = ProcessedImageField(verbose_name=_("thumbnail"), storage=post_image_storage, upload_to=upload_to_post_image_directory, blank=False, null=True, format="JPEG", options={"quality": 50}, processors=[ResizeToFit(width=1024, upscale=False)])
    media = GenericRelation(PostMedia)
    image_view_type = (
        ("regularimage", "regularimage"),
        ("gridimages", "gridimages"),
    )
    viewtype = models.CharField(max_length=24, choices=image_view_type, default="")

创建PostImage对象时触发错误：

null value in column "width" violates not null constraint.

因为后续逻辑需要用到width和height的计算值，不想把这两个字段设为null=True。测试时设为null=True后能创建对象，但width和height字段依然没有值。

错误栈详情：

django.db.utils.IntegrityError: null value in column "width" of relation "myapi_posts_postimage" violates not-null constraint
my-api-webserver-0  | DETAIL:  Failing row contains (1, posts/9957dc49-639b-490e-87f5-54cab35c0229/582a8f62-86d5-4c3d-a1..., null, null, 01d2d1ce86aa7007f1cecad2259394163b95987a850e6c2d6a48c322a2dc8df0, posts/9957dc49-639b-490e-87f5-54cab35c0229/33d944aa-ded1-49cb-a9..., regularimage, 1).

这个问题是django-imagekit的ProcessedImageField未自动填充width/height字段导致的，可按以下方法解决：

• 调整字段定义顺序
  将width和height字段移到image字段前面。Django模型字段的定义顺序会影响赋值逻辑，imagekit需要先存在对应的宽高字段，才能正确写入计算值。调整后的核心代码：

  class PostImage(models.Model):
      post = models.ForeignKey(Post, on_delete=models.CASCADE, null=True, related_name="images")
      width = models.PositiveIntegerField(editable=False, null=False, blank=False)
      height = models.PositiveIntegerField(editable=False, null=False, blank=False)
      image = ProcessedImageField(verbose_name=_("image"), storage=post_image_storage, upload_to=upload_to_post_image_directory, width_field="width", height_field="height", blank=False, null=True, format="JPEG", options={"quality": 100}, processors=[ResizeToFit(width=1024, upscale=False)])
      # 剩余字段保持不变
  

• 确保触发imagekit处理逻辑
  如果是手动创建对象，不要直接赋值image后就save，而是先通过imagekit的工具方法处理图片再保存：

  from imagekit.utils import process_image
  
  # 假设image_file是上传的文件对象
  post_image = PostImage(post=target_post)
  # 手动处理图片
  processed_file = process_image(image_file, processors=[ResizeToFit(width=1024, upscale=False)], format="JPEG", options={"quality": 100})
  # 保存处理后的图片，此时width/height会自动填充
  post_image.image.save(processed_file.name, processed_file)
  post_image.save()
  

• 排查自定义存储干扰
  暂时替换post_image_storage为Django默认存储，测试是否是自定义存储的save方法干扰了imagekit的处理流程。如果是，需要调整自定义存储的逻辑，确保不阻断imagekit的图片处理步骤。

• 用信号手动填充宽高
  若以上方法无效，通过Django的pre_save信号在保存前手动计算图片尺寸：

  from django.db.models.signals import pre_save
  from django.dispatch import receiver
  from PIL import Image
  
  @receiver(pre_save, sender=PostImage)
  def fill_image_dimensions(sender, instance, **kwargs):
      if instance.image and not instance.width:
          with instance.image.open() as img_file:
              img = Image.open(img_file)
              instance.width, instance.height = img.size
  

内容的提问来源于stack exchange，提问作者Earthling