Saving object creation datetime using Pynamodb - amazon-dynamodb

Question

I want to update the creation_datetime only when a new object is created and update the last_update_datetime on every update while using the save method. default_for_new is updating time even when the existing object is updating. any alternatives?
below is the sample code I tried
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

The best way you can do this is overriding the update method of your Object class. This is an example:
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)
We just need to append an action to the actions list asking the timestamp update. Then call the parent's update method. Now you can forget about the timestamps and use the update method as you would.
item = SalesItem.get('pynamodb-test')
# do your updates here
item.update(actions=[
SalesItem.sales.set(item.sales + 1)
])