Finding objects according to related objects - django

Question

I am trying to build a messaging application with Django. The reason I don’t use postman is that I need messaging between other objects than users and I don’t need most of the postman’s features.
Here are my models:
class Recipient(models.Model):
...
def get_unread_threads():
see below...
class Thread(models.Model):
author = models.ForeignKey(Recipient, related_name='initiated_threads')
recipients = models.ManyToManyField(
Tribe,
related_name='received_thread',
through='ThreadReading')
subject = models.CharField(max_length=64)
class Meta:
app_label = 'game'
class Message(models.Model):
author = models.ForeignKey(Recipient, related_name='sent_messages')
date_add = models.DateTimeField(auto_now_add=True)
date_edit = models.DateTimeField(blank=True)
content = models.TextField(max_length=65535)
thread = models.ForeignKey(Thread)
class Meta:
get_latest_by = 'date'
class ThreadReading(models.Model):
thread = models.ForeignKey(Thread)
recipient = models.ForeignKey(Recipient)
date_last_reading = models.DateTimeField(auto_now=True)
My problem is about get_unread_threads. I can’t really find out how to do that. Here is a first try:
def get_unread_threads(self):
"""
return a queryset corresponding to the threads
which at least one message is unread by the recipient
"""
try:
query = self.received_thread.filter(
message__latest__date_add__gt=\
self.threadreading_set.get(thread_id=F('id')).date_last_reading)
except ObjectDoesNotExist:
query = None
return query
But obviously it doesn’t work because lookup can’t follow method latest.

Answer 1

Here you go:
# Get all the readings of the user
thread_readings = recipient.threadreading_set.all()
# Build a query object including all messages who's last reading is after the
# last edit that was made AND whose thread is the thread of the current
# iteration's thread reading
q = models.Q()
for thread_reading in thread_readings:
q = q | models.Q(
models.Q(
date_edit__lte=thread_reading.date_last_reading
& models.Q(
thread=thread_reading.thread
)
)
)
# Get a queryset of all the messages, including the threads (via a JOIN)
queryset = Message.objects.select_related('thread')
# Now, exclude from the queryset every message that matches the above query
# (edited prior to last reading) OR that is in the list of last readings
queryset = queryset.exclude(
q | models.Q(
thread__pk__in=[thread_reading.pk for thread_reading in thread_readings]
)
)
# Make an iterator (to pretend that this is efficient) and return a generator of it
iterator = queryset.iterator()
return (message.thread for message in iterator)
:)
Now, don't ever actually do this - rethink your models. I would read a book called "Object Oriented Analysis and Design with Applications". It'll teach you a great deal about how to think when you're data modelling.