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?
用David Cramer的解决方案怎么样:
http://cramer.io/2010/12/06/tracking-changes-to-fields-in-django/
我曾经这样成功地使用过:
@track_data('name')
class Mode(models.Model):
name = models.CharField(max_length=5)
mode = models.CharField(max_length=5)
def save(self, *args, **kwargs):
if self.has_changed('name'):
print 'name changed'
# OR #
@classmethod
def post_save(cls, sender, instance, created, **kwargs):
if instance.has_changed('name'):
print "Hooray!"
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])
最好的方法是使用pre_save信号。在09年这个问题被提出并回答的时候，可能还没有这个选项，但今天看到这个问题的人应该这样做:
@receiver(pre_save, sender=MyModel)
def do_something_if_changed(sender, instance, **kwargs):
try:
obj = sender.objects.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
