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

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

其他回答

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。

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。

修改@ivanperelivskiy的回答:

@property
def _dict(self):
    ret = {}
    for field in self._meta.get_fields():
        if isinstance(field, ForeignObjectRel):
            # foreign objects might not have corresponding objects in the database.
            if hasattr(self, field.get_accessor_name()):
                ret[field.get_accessor_name()] = getattr(self, field.get_accessor_name())
            else:
                ret[field.get_accessor_name()] = None
        else:
            ret[field.attname] = getattr(self, field.attname)
    return ret

这里使用了django 1.10的公共方法get_fields。这使得代码更经得起未来的考验,但更重要的是还包括了外键和edititable =False的字段。

作为参考,这里是.fields的实现

@cached_property
def fields(self):
    """
    Returns a list of all forward fields on the model and its parents,
    excluding ManyToManyFields.

    Private API intended only to be used by Django itself; get_fields()
    combined with filtering of field properties is the public API for
    obtaining this field list.
    """
    # For legacy reasons, the fields property should only contain forward
    # fields that are not private or with a m2m cardinality. Therefore we
    # pass these three filters as filters to the generator.
    # The third lambda is a longwinded way of checking f.related_model - we don't
    # use that property directly because related_model is a cached property,
    # and all the models may not have been loaded yet; we don't want to cache
    # the string reference to the related_model.
    def is_not_an_m2m_field(f):
        return not (f.is_relation and f.many_to_many)

    def is_not_a_generic_relation(f):
        return not (f.is_relation and f.one_to_many)

    def is_not_a_generic_foreign_key(f):
        return not (
            f.is_relation and f.many_to_one and not (hasattr(f.remote_field, 'model') and f.remote_field.model)
        )

    return make_immutable_fields_list(
        "fields",
        (f for f in self._get_fields(reverse=False)
         if is_not_an_m2m_field(f) and is_not_a_generic_relation(f) and is_not_a_generic_foreign_key(f))
    )

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

虽然这实际上并没有回答你的问题,但我会以不同的方式来讨论这个问题。

成功保存本地副本后,只需清除remote_image字段。然后在保存方法中,只要remote_image不为空,就可以随时更新图像。

如果您想保留对url的引用,您可以使用一个不可编辑的布尔字段来处理缓存标志,而不是使用remote_image字段本身。