在我的模型中我有:
class Alias(MyBaseModel):
remote_image = models.URLField(
max_length=500, null=True,
help_text='''
A URL that is downloaded and cached for the image.
Only used when the alias is made
'''
)
image = models.ImageField(
upload_to='alias', default='alias-default.png',
help_text="An image representing the alias"
)
def save(self, *args, **kw):
if (not self.image or self.image.name == 'alias-default.png') and self.remote_image :
try :
data = utils.fetch(self.remote_image)
image = StringIO.StringIO(data)
image = Image.open(image)
buf = StringIO.StringIO()
image.save(buf, format='PNG')
self.image.save(
hashlib.md5(self.string_id).hexdigest() + ".png", ContentFile(buf.getvalue())
)
except IOError :
pass
这在remote_image第一次改变的时候工作得很好。
当有人修改了别名上的remote_image时，我如何获取一个新的图像?其次，是否有更好的方法来缓存远程映像?
这是Chris Pratt的答案的一个版本，通过使用事务块和select_for_update()来防止竞争条件，同时牺牲性能。
@receiver(pre_save, sender=MyModel)
@transaction.atomic
def do_something_if_changed(sender, instance, **kwargs):
try:
obj = sender.objects.select_for_update().get(pk=instance.pk)
except sender.DoesNotExist:
pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
else:
if not obj.some_field == instance.some_field: # Field has changed
# do something
最佳的解决方案可能是在保存模型实例之前不包括额外的数据库读取操作，也不包括任何进一步的django库。这就是为什么拉弗斯特的解决方案更可取。在管理站点的上下文中，可以简单地覆盖save_model-方法，并调用表单的has_changed方法，就像上面Sion的回答一样。你得到了类似这样的东西，利用Sion的例子设置，但使用changed_data来获得每一个可能的变化:
class ModelAdmin(admin.ModelAdmin):
fields=['name','mode']
def save_model(self, request, obj, form, change):
form.changed_data #output could be ['name']
#do somethin the changed name value...
#call the super method
super(self,ModelAdmin).save_model(request, obj, form, change)
覆盖save_model:
https://docs.djangoproject.com/en/1.10/ref/contrib/admin/#django.contrib.admin.ModelAdmin.save_model
为字段内置changed_data-method:
https://docs.djangoproject.com/en/1.10/ref/forms/api/#django.forms.Form.changed_data
我对@iperelivskiy的解决方案的看法是:在大规模的情况下，为每个__init__创建_initial字典是昂贵的，而且大多数时候是不必要的。我稍微改变了mixin，这样它只在你显式地告诉它这样做(通过调用instance.track_changes)时才记录更改:
from typing import KeysView, Optional
from django.forms import model_to_dict
class TrackChangesMixin:
_snapshot: Optional[dict] = None
def track_changes(self):
self._snapshot = self.as_dict
@property
def diff(self) -> dict:
if self._snapshot is None:
raise ValueError("track_changes wasn't called, can't determine diff.")
d1 = self._snapshot
d2 = self.as_dict
diffs = [(k, (v, d2[k])) for k, v in d1.items() if str(v) != str(d2[k])]
return dict(diffs)
@property
def has_changed(self) -> bool:
return bool(self.diff)
@property
def changed_fields(self) -> KeysView:
return self.diff.keys()
@property
def as_dict(self) -> dict:
return model_to_dict(self, fields=[field.name for field in self._meta.fields])
这是Chris Pratt的答案的一个版本，通过使用事务块和select_for_update()来防止竞争条件，同时牺牲性能。
@receiver(pre_save, sender=MyModel)
@transaction.atomic
def do_something_if_changed(sender, instance, **kwargs):
try:
obj = sender.objects.select_for_update().get(pk=instance.pk)
except sender.DoesNotExist:
pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
else:
if not obj.some_field == instance.some_field: # Field has changed
# do something
