Question

I’m trying to design a gmail like thread messaging system on Google App Engine by using Django-nonrel. I’ve watched “Building Scalable, Complex Apps on App Engine” and read the following questions on SO.

App Engine Messaging System with Message Status – Design Pattern
database design in google app engine

I like the idea Kevin pointed at (App Engine Messaging System with Message Status – Design Pattern), so my current models are like these:

class Message(models.Model):
    sender = models.ForeignKey(User)
    content = models.TextField()
    thread_id = models.IntegerField()

class MessageReceivers(models.Model):
    message = models.ForeignKey(Message)
    users = ListField(models.CharField())
    status = models.IntegerField()  # 1 unread, 2 read ....

ListField on Django-nonrel is from here:
http://www.allbuttonspressed.com/blog/django/2010/03/Updates-on-djangoappengine

And here is my problem, I don’t know how to achieve to display a count of messages in each thread like gmail. Let me clarify my point.
When a user, Sirius, sends a message to Harry and Ron. Models (simplified for explanation) should be like these:

Message
id = 1
sender = Sirius
thread_id = 1

MessageReceivers
message = 1
users = [Harry, Ron]

Then, Harry replies to Sirius, but for some reason he doesn’t include Ron:

Message
id = 2
sender = Harry
thread_id = 1

MessageReceivers
message = 2
users = [Sirius]

And Sirius replies to Harry too, but he includes Ron again:

Message
id = 3
sender = Sirius
thread_id = 1

MessageReceivers
message = 3
users = [Haary, Ron]

And when Sirius and Harry see their inbox, there is the thread with a count of messages (3). And Ron sees his inbox there is the thread with a count of messages (2). How should I count those numbers? Should I make a new model to keep them?

Thanks in advance,
Yoo

EDIT
As Kevin introduced me to a Thread model and I realized that I don’t need a relation index model for my requirements, so my current models look like the following.

class Message(models.Model):
    sender = models.ForeignKey(User)
    content = models.TextField()

class Thread(models.Model):
    user = models.ForeignKey(User)
    messages = ListField(models.CharField())    # array of message id
    lastmodified = models.DateTimeField()
    # could have a status field for read or unread, not sure for now

I might find difficulties in these models in further development, but I would go with them for the time being 🙂

Answer 1

Try this:

from collections import defaultdict

messages = MessageReceivers.objects.filter(users='Harry')

message_threads = []
for m in messages:
  message_threads.append(m.message.thread_id)

counted_threads = defaultdict(int)

for curr in message_threads:
    counted_threads[curr] += 1

This will give you a dictionary of thread_id’s, with their frequency of occurrence.

For example:

message_threads = [‘a’,’b’,’c’,’a’,’a’,’b’,’a’]

will yield

counted_threads == defaultdict(, {‘a’: 4, ‘c’: 1, ‘b’: 2})