修复若干bug，优化若干代码

apps/work/signals.py

@@ -0,0 +1,16 @@
+from django.db.models.signals import pre_save,pre_delete,post_init,post_save,pre_init
+from django.dispatch import receiver
+
+from .models import Works
+
+
+# 修改时,保存图片并删除旧图
+# @receiver(post_init, sender=Works)
+# def file_path(sender, instance, **kwargs):
+#     instance._current_image = instance.image
+
+
+@receiver(pre_save, sender=Works)
+def delete_old_image(sender, instance, **kwargs):
+    print(instance.img)
+    instance.img.delete(save=True)