Question

My app-engine data model is defined as:

class Event(db.Model):
    title = db.LinkProperty();
    refresh_interval = db.IntegerProperty(); 

class EventSchedule(db.Model):
    event = db.ReferenceProperty(Event)     # referencing the event
    refresh_date = db.DateTimeProperty();   # date & time of last successful refresh 

I’d like to fetch all EventSchedule items based on the criteria of:

event_schedule.refresh_date + event.refresh_interval >= now

In SQL-land, it would look something like:
DATE_ADD(event_schedule.refresh_date, event.refreshInterval) >= now()

Is this kind of join operation feasible with the data store?

Answer 1

This join operation is not possible with the GAE data store, and is highly inefficient on normal SQL databases as well (since it requires a scan of the entire event_schedule table).

You should instead store a next_refresh DateTimeProperty in your EventSchedule entities, initialized to be refresh_date + refresh_interval when the entity is created, and simply test for next_refresh >= now.