How to model social graph like flockDB in Google App Engine

FlockDB provides a very nice way to model a social graph: There are only two entities in the system, User and Friendship. where User represents vertices of the graph and friendship represents edges between two users. Like this:

User(id, first_name, last_name, birth_data, registration_timestamp)

Friendship(id, user1_id, user2_id, depth, other friendship params ...)

There are a number of advantages of this approach

• You can specify params for friendship like friend, close friend, in a relationship
• You can choose whether your friendship are d开发者_如何学编程irectional or not.

My question is How to implement this with DataStore provided by Google AppEngine. here is my first attempt, but i dont understand how really to assign keys to the friendship entities.

class User(db.Model):
  name = db.StringProperty()
  age = db.IntegerProperty()
  registeration_ts = db.DateTimeProperty(auto_now_add=True)

class FriendShip(db.Model):
  user1 = db.Key()   // key to some user in user1
  user2 = db.Key()   // key to some user in user2
  creation_ts = db.DateTimeProperty(auto_now_add=True)
  updated_ts = db.DateTimeProperty(auto_now_add=True)

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one way would be to model like this:

class User(db.Model):
 name = db.StringProperty()
 friends = db.SelfReferenceProperty(collection_name="subscribers")

This would be a simple model to represent a user and friends. But i think it will have two disadvatnages:

1. Since friendship is not an entity, we can't assign properties to it
2. since now a collection is maintained, i don't have idea how fast it will to query like fetching friends of friends.

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Instead of db.Key() (which is a value, not a property class), use db.ReferenceProperty(User).