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?
这是另一种方法。
class Parameter(models.Model):
def __init__(self, *args, **kwargs):
super(Parameter, self).__init__(*args, **kwargs)
self.__original_value = self.value
def clean(self,*args,**kwargs):
if self.__original_value == self.value:
print("igual")
else:
print("distinto")
def save(self,*args,**kwargs):
self.full_clean()
return super(Parameter, self).save(*args, **kwargs)
self.__original_value = self.value
key = models.CharField(max_length=24, db_index=True, unique=True)
value = models.CharField(max_length=128)
根据文档:验证对象
full_clean()执行的第二步是调用Model.clean()。应该重写此方法,以便在模型上执行自定义验证。
此方法应用于提供自定义模型验证,并根据需要修改模型上的属性。例如,你可以使用它来自动为一个字段提供一个值,或者进行验证,需要访问多个字段:
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
@ivanlivski的mixin很棒。
我把它扩展到
确保它适用于Decimal字段。
公开属性以简化使用
更新后的代码可在这里:
https://github.com/sknutsonsf/python-contrib/blob/master/src/django/utils/ModelDiffMixin.py
为了帮助刚接触Python或Django的人,我将给出一个更完整的示例。
这种特殊用法是从数据提供者获取一个文件,并确保数据库中的记录反映该文件。
我的模型对象:
class Station(ModelDiffMixin.ModelDiffMixin, models.Model):
station_name = models.CharField(max_length=200)
nearby_city = models.CharField(max_length=200)
precipitation = models.DecimalField(max_digits=5, decimal_places=2)
# <list of many other fields>
def is_float_changed (self,v1, v2):
''' Compare two floating values to just two digit precision
Override Default precision is 5 digits
'''
return abs (round (v1 - v2, 2)) > 0.01
加载文件的类有这些方法:
class UpdateWeather (object)
# other methods omitted
def update_stations (self, filename):
# read all existing data
all_stations = models.Station.objects.all()
self._existing_stations = {}
# insert into a collection for referencing while we check if data exists
for stn in all_stations.iterator():
self._existing_stations[stn.id] = stn
# read the file. result is array of objects in known column order
data = read_tabbed_file(filename)
# iterate rows from file and insert or update where needed
for rownum in range(sh.nrows):
self._update_row(sh.row(rownum));
# now anything remaining in the collection is no longer active
# since it was not found in the newest file
# for now, delete that record
# there should never be any of these if the file was created properly
for stn in self._existing_stations.values():
stn.delete()
self._num_deleted = self._num_deleted+1
def _update_row (self, rowdata):
stnid = int(rowdata[0].value)
name = rowdata[1].value.strip()
# skip the blank names where data source has ids with no data today
if len(name) < 1:
return
# fetch rest of fields and do sanity test
nearby_city = rowdata[2].value.strip()
precip = rowdata[3].value
if stnid in self._existing_stations:
stn = self._existing_stations[stnid]
del self._existing_stations[stnid]
is_update = True;
else:
stn = models.Station()
is_update = False;
# object is new or old, don't care here
stn.id = stnid
stn.station_name = name;
stn.nearby_city = nearby_city
stn.precipitation = precip
# many other fields updated from the file
if is_update == True:
# we use a model mixin to simplify detection of changes
# at the cost of extra memory to store the objects
if stn.has_changed == True:
self._num_updated = self._num_updated + 1;
stn.save();
else:
self._num_created = self._num_created + 1;
stn.save()
这是Chris Pratt的答案的一个版本,通过使用事务块和select_for_update()来防止竞争条件,同时牺牲性能。
@receiver(pre_save, sender=MyModel)
@transaction.atomic
def do_something_if_changed(sender, instance, **kwargs):
try:
obj = sender.objects.select_for_update().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