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I'm trying to create a site which is quite similar to Twitter. Users will be able to post messages. And users will be able to 'follow' each other. On the homepage, they see the messages from the users they follow, sorted by time.

How do I go about creating the appengine models for this?

In a traditional relational DB, i guess it would be something like this:

Database 'user':

  • id
  • username

Database 'follows':

  • user_id
  • follow_id

Database 'messages':

  • user_id
  • message

And the query will be something like:

SELECT * FROM messages m, follows f WHERE m.user_id = f.follow_id AND f.user_id = current_user_id

I guess i was clear with the example above. How do I replicate this in Google App Engine?

Shrihari
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2 Answers2

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There is a useful presentation at Google I/O a while back by Brett Slatkin which describes building a scalable twitter-like microblog app, and deals with this very question at length: http://www.google.com/events/io/2009/sessions/BuildingScalableComplexApps.html

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REVISED:

class AppUser(db.Model):
    user_id = db.UserProperty()
    username = db.StringProperty()
    following = db.ListProperty(db.Key) # list of AppUser keys

class Message(db.Model):
    sender = db.ReferenceProperty(AppUser)
    body = db.TextProperty()

You would then query the results in two steps:

message_list = []
for followed_user in current_user.following:
    subresult = db.GqlQuery("SELECT __key__ FROM Message WHERE sender = :1", followed_user)
    message_list.extend(subresult)

results = Message.get(message_list)

(with 'current_user' being the 'AppUser' entity corresponding with your active user)

Kevin P
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  • But this wouldn't work in this scenario:- 1) User A posts a message, 2) User B follows User A, 3) Now, User B will not be able to see the message posted in (1).. Also, 1) User A follows User B 2) User B posts a message, 3) User A un-follows User B, 4) User A will still see the message posted in (2).. Am I right? – Shrihari Jun 09 '11 at 04:32
  • You are correct, it wouldn't accommodate historical/retro-active following. I've posted a revised solution that would, I think, accomplish what you are aiming for. – Kevin P Jun 10 '11 at 21:15