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?
现在直接回答:检查字段的值是否已经改变的一种方法是在保存实例之前从数据库获取原始数据。想想这个例子:
class MyModel(models.Model):
f1 = models.CharField(max_length=1)
def save(self, *args, **kw):
if self.pk is not None:
orig = MyModel.objects.get(pk=self.pk)
if orig.f1 != self.f1:
print 'f1 changed'
super(MyModel, self).save(*args, **kw)
同样的事情也适用于处理表单。您可以在ModelForm的clean或save方法中检测到它:
class MyModelForm(forms.ModelForm):
def clean(self):
cleaned_data = super(ProjectForm, self).clean()
#if self.has_changed(): # new instance or existing updated (form has data to save)
if self.instance.pk is not None: # new instance only
if self.instance.f1 != cleaned_data['f1']:
print 'f1 changed'
return cleaned_data
class Meta:
model = MyModel
exclude = []
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!
有时我想检查多个共享这些字段的模型上相同特定字段的更改，因此我定义了这些字段的列表并使用一个信号。在这种情况下，只有当某些东西发生了变化，或者条目是新的时，地理编码才会寻址:
from django.db.models.signals import pre_save
from django.dispatch import receiver
@receiver(pre_save, sender=SomeUserProfileModel)
@receiver(pre_save, sender=SomePlaceModel)
@receiver(pre_save, sender=SomeOrganizationModel)
@receiver(pre_save, sender=SomeContactInfoModel)
def geocode_address(sender, instance, *args, **kwargs):
input_fields = ['address_line', 'address_line_2', 'city', 'state', 'postal_code', 'country']
try:
orig = sender.objects.get(id=instance.id)
if orig:
changes = 0
for field in input_fields:
if not (getattr(instance, field)) == (getattr(orig, field)):
changes += 1
if changes > 0:
# do something here because at least one field changed...
my_geocoder_function(instance)
except:
# do something here because there is no original, or pass.
my_geocoder_function(instance)
只编写一次并使用“@receiver”附加当然胜过重写多个模型保存方法，但也许其他人有更好的想法。
