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?


当前回答

作为SmileyChris回答的扩展,您可以为last_updated的模型添加一个datetime字段,并在检查更改之前为您将让它达到的最大年龄设置某种限制

其他回答

你可以使用django-model-changes来做到这一点,而不需要额外的数据库查询:

from django.dispatch import receiver
from django_model_changes import ChangesMixin

class Alias(ChangesMixin, MyBaseModel):
   # your model

@receiver(pre_save, sender=Alias)
def do_something_if_changed(sender, instance, **kwargs):
    if 'remote_image' in instance.changes():
        # do something

本质上,你想要重写模型的__init__方法。模型,以便保留原始值的副本。这使得您不必再执行另一个DB查找(这总是一件好事)。

    class Person(models.Model):
        name = models.CharField()

        __original_name = None

        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.__original_name = self.name

        def save(self, force_insert=False, force_update=False, *args, **kwargs):
            if self.name != self.__original_name:
                # name changed - do something here

            super().save(force_insert, force_update, *args, **kwargs)
            self.__original_name = self.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

我找到了这个包django-lifecycle。 它使用django信号来定义@hook装饰器,非常健壮可靠。我用过它,它是一种幸福。

我使用以下mixin:

from django.forms.models import model_to_dict


class ModelDiffMixin(object):
    """
    A model mixin that tracks model fields' values and provide some useful api
    to know what fields have been changed.
    """

    def __init__(self, *args, **kwargs):
        super(ModelDiffMixin, self).__init__(*args, **kwargs)
        self.__initial = self._dict

    @property
    def diff(self):
        d1 = self.__initial
        d2 = self._dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if v != d2[k]]
        return dict(diffs)

    @property
    def has_changed(self):
        return bool(self.diff)

    @property
    def changed_fields(self):
        return self.diff.keys()

    def get_field_diff(self, field_name):
        """
        Returns a diff for field if it's changed and None otherwise.
        """
        return self.diff.get(field_name, None)

    def save(self, *args, **kwargs):
        """
        Saves model and set initial state.
        """
        super(ModelDiffMixin, self).save(*args, **kwargs)
        self.__initial = self._dict

    @property
    def _dict(self):
        return model_to_dict(self, fields=[field.name for field in
                             self._meta.fields])

用法:

>>> p = Place()
>>> p.has_changed
False
>>> p.changed_fields
[]
>>> p.rank = 42
>>> p.has_changed
True
>>> p.changed_fields
['rank']
>>> p.diff
{'rank': (0, 42)}
>>> p.categories = [1, 3, 5]
>>> p.diff
{'categories': (None, [1, 3, 5]), 'rank': (0, 42)}
>>> p.get_field_diff('categories')
(None, [1, 3, 5])
>>> p.get_field_diff('rank')
(0, 42)
>>>

Note

请注意,此解决方案仅在当前请求的上下文中工作良好。因此,它主要适用于简单的情况。在并发环境中,多个请求可以同时操作同一个模型实例,您肯定需要一种不同的方法。