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?
我扩展了@livskiy的mixin，如下所示:
class ModelDiffMixin(models.Model):
"""
A model mixin that tracks model fields' values and provide some useful api
to know what fields have been changed.
"""
_dict = DictField(editable=False)
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.
"""
object_dict = model_to_dict(self,
fields=[field.name for field in self._meta.fields])
for field in object_dict:
# for FileFields
if issubclass(object_dict[field].__class__, FieldFile):
try:
object_dict[field] = object_dict[field].path
except :
object_dict[field] = object_dict[field].name
# TODO: add other non-serializable field types
self._dict = object_dict
super(ModelDiffMixin, self).save(*args, **kwargs)
class Meta:
abstract = True
DictField为:
class DictField(models.TextField):
__metaclass__ = models.SubfieldBase
description = "Stores a python dict"
def __init__(self, *args, **kwargs):
super(DictField, self).__init__(*args, **kwargs)
def to_python(self, value):
if not value:
value = {}
if isinstance(value, dict):
return value
return json.loads(value)
def get_prep_value(self, value):
if value is None:
return value
return json.dumps(value)
def value_to_string(self, obj):
value = self._get_val_from_obj(obj)
return self.get_db_prep_value(value)
可以通过在模型中扩展它来使用它
一个_dict字段将在同步/迁移时添加，该字段将存储对象的状态
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
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。
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 = []
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
