使用Pynamodb保存对象创建日期时间

Question

我想仅在创建新对象时更新creation_datetime，并在使用save方法的每次更新时更新last_update_datetime。即使现有对象正在更新，default_for_new也是更新时间。替代品吗?

下面是我尝试的示例代码

from pynamodb.models import Model
from pynamodb.attributes import  UTCDateTimeAttribute
def current_datetime():
from datetime import datetime
return datetime.now()

class AbstractDateTimeModel(Model):
creation_datetime = UTCDateTimeAttribute(default_for_new=current_datetime)
last_update_datetime = UTCDateTimeAttribute(default=current_datetime)
class Meta(object):
abstract = True

Answer 1

最好的方法是重写Object类的update方法。下面是一个示例:

from datetime import datetime, timezone
from pynamodb.models import Model
from pynamodb.settings import OperationSettings
from pynamodb.attributes import UTCDateTimeAttribute, UnicodeAttribute, NumberAttribute

def get_current_time_utc():
return datetime.now(timezone.utc)

class SalesItem(Model):
class Meta:
table_name = 'monthlySales'
id = UnicodeAttribute(hash_key=True)
month= UnicodeAttribute()
sales= NumberAttribute()
createDateTime = UTCDateTimeAttribute(default_for_new=get_current_time_utc)
updateDateTime = UTCDateTimeAttribute(default_for_new=get_current_time_utc)

# overriding the method to add timestamp on update
def update(self, actions=[], condition=None, settings=OperationSettings.default):
actions.append(SalesItem.updateDateTime.set(get_current_time_utc()))
Model.update(self, actions, condition, settings)

我们只需要在动作列表中添加一个请求时间戳更新的动作。然后调用父类的更新方法。现在，您可以忘记时间戳，并像往常一样使用更新方法。

item = SalesItem.get('pynamodb-test')
# do your updates here
item.update(actions=[
SalesItem.sales.set(item.sales + 1)
])