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?
自从Django 1.8发布以来，你可以使用from_db类方法来缓存remote_image的旧值。然后在保存方法中，您可以比较字段的旧值和新值，以检查该值是否已更改。
@classmethod
def from_db(cls, db, field_names, values):
new = super(Alias, cls).from_db(db, field_names, values)
# cache value went from the base
new._loaded_remote_image = values[field_names.index('remote_image')]
return new
def save(self, force_insert=False, force_update=False, using=None,
update_fields=None):
if (self._state.adding and self.remote_image) or \
(not self._state.adding and self._loaded_remote_image != self.remote_image):
# If it is first save and there is no cached remote_image but there is new one,
# or the value of remote_image has changed - do your stuff!
改进所有领域的@josh答案:
class Person(models.Model):
name = models.CharField()
def __init__(self, *args, **kwargs):
super(Person, self).__init__(*args, **kwargs)
self._original_fields = dict([(field.attname, getattr(self, field.attname))
for field in self._meta.local_fields if not isinstance(field, models.ForeignKey)])
def save(self, *args, **kwargs):
if self.id:
for field in self._meta.local_fields:
if not isinstance(field, models.ForeignKey) and\
self._original_fields[field.name] != getattr(self, field.name):
# Do Something
super(Person, self).save(*args, **kwargs)
澄清一下，getattr的工作是获取像person.name这样带字符串的字段(即getattr(person， "name"))
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
