In App Engine for Python, is it possible to persist a class with another object nested inside it?

Question

In App Engine for Python, is there anything like Objectify (Java Library) where I can easily embed a class within another and save it to the datastore?

This class would be modeled like the following example where a Venue contain a Location object. I want to persist this as one nested object, as well as be able to query by fields in the embedded object.

class Location():
city = db.StringProperty()
state = db.StringProperty()

class Venue(db.Model):
name = db.StringProperty()
location = Location()

Here is information on how it works in Objectify in App Engine for Java.
http://code.google.com/p/objectify-appengine/wiki/IntroductionToObjectify#@Embedded

Is this possible using Python?

Answer 1

Consider using Reference properties. Ie store a Location object as its own entity and incorporate that location into the Venue object by reference.

class Location():
  city = db.StringProperty()
  state = db.StringProperty()

class Venue(db.Model):
  name = db.StringProperty()
  location = db.ReferenceProperty(Location)

Then, if you want to transact on a Location and Venue at the same time, use datastore transactions.

EDIT: To query fields in the 'contained' object, use datastore "back references". Ie the fact that Venue contains a reference to Location means Location also contains references to Venues. See: http://code.google.com/appengine/docs/python/datastore/datamodeling.html#References

Answer 2

Not currently, but the NDB library supports embedding models within one another either by serializing them as Protocol Buffers, or by nesting their properties (Objectify fashion).