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?
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
@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()
用David Cramer的解决方案怎么样:
http://cramer.io/2010/12/06/tracking-changes-to-fields-in-django/
我曾经这样成功地使用过:
@track_data('name')
class Mode(models.Model):
name = models.CharField(max_length=5)
mode = models.CharField(max_length=5)
def save(self, *args, **kwargs):
if self.has_changed('name'):
print 'name changed'
# OR #
@classmethod
def post_save(cls, sender, instance, created, **kwargs):
if instance.has_changed('name'):
print "Hooray!"
