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。
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:
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
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。
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
