best practice for tracking new/seen items
There's probably a few ways to go about this, and using Google Cloud SQL would probably make your life easier for this. What you could do though is add a BooleanProperty to your entity types for example, named 'New', that would be set to 'True' by default. You would need to create an algorithm that would check through the datastore to return you any entity who's 'New' BooleanProperty is set to True, and then set it to False once it's returned.
So I'm making productivity app for myself and ppl around me. There will be a lots of different stuff related to different type groups or individual persons for different types of reasons.
Is there some best practice approach, algorythm or guidelines to do this properly that I should know before I screw everything up?
If it matters, I'm using GAE, Python, NDB.
CLARIFICATION EDIT:

someone contributed to household's savings account
Someone posted a picture to album of the party that I attended
somone from household added someting to shopping list
i received a direct message
someone uploaded a documentg to project workspace etc

I need to keep track of which items are new and which have been seen.