Thank you so much! That was a very helpful explanation. And it worked
perfectly!!!
Just, one more question please. Suppose, the user wants to send a message
to a group of users, how do I find the thread in this situation?
Eg: user1 wants to send a message to both the user2 and user3. How to find
the thread, in this case '*t3*'?
>>> t1 = Thread.objects.get(id=1)
>>> t1
[<User: user1>,<User: user3>]
>>> t2 = Thread.objects.get(id=2)
>>> t2
[<User: user1>,<User: user2>]
>>> t3 = Thread.objects.get(id=3)
>>> t3
[<User: user1>,<User: user2>,<User: user3>]
On Wed, Dec 4, 2013 at 5:41 PM, Tom Evans <tevans...@googlemail.com> wrote:
> On Wed, Dec 4, 2013 at 10:24 AM, Aamu Padi <aamup...@gmail.com> wrote:
> > How do I check whether there is a thread containing only the sender
> (user 1)
> > and the recipient (user 2), and no other users.
> >
> > models.py
> >
> > class Thread(models.Model):
> > user = models.ManyToManyField(User)
> > is_hidden = models.ManyToManyField(User,
> > related_name='hidden_thread', blank=True)
> >
> > class Message(models.Model):
> > thread = models.ForeignKey(Thread)
> > sent_date = models.DateTimeField(default=datetime.now)
> > sender = models.ForeignKey(User)
> > body = models.TextField()
> > is_hidden = models.ManyToManyField(User,
> > related_name='hidden_message', blank=True)
> >
> > I tried
> >
> > >>> Thread.objects.filter(user=user1&user2)
> > >>> Thread.objects.filter(user=user1|user2)
> > # Both gave me an error: Unsupported operand.
> >
> > # Then this
> > >>> Thread.objects.filter(Q(user=user1) & Q(user=user2))
> > # Which gave me no threads at all.
> >
> > # Then this
> > >>> Thread.objects.filter(Q(user=user1) | Q(user=user2)).distinct()
> > # Gave me threads of both the users.
> >
> > What I want is to check the thread, with the specified users only.
> Suppose,
> > User 1 wants to send a message to User 2. What I want is, first check
> > whether there is a thread between both the users. If there is, get that
> > thread, or else create a new one. How is it possible? What is the best
> way
> > to do it. Please help me. Thank you.
>
> I hope you understand SQL a little, as it will be required to follow
> this explanation.
>
> The condition "Thread.objects.filter(user=..)" goes across a join to
> the users<->threads "through" table, which has columns (id, thread_id,
> user_id).
>
> When you specify conditions in a "filter()" call, each of the
> conditions specified in that call is AND'ed together (the equivalent
> of what you were doing in the "Q(user=user1) & Q(user=user2)", django
> would do that for you). However, what does that mean, in terms of SQL?
>
> Well, it means "find me rows from the users<->threads through table
> that have user_id=user1.pk and user_id=user2.pk". Obviously, this will
> never find results unless user1==user2.
>
> So, you need to specify that you want to join twice to that table. You
> do this by specifying the conditions for each join in a separate
> "filter()" call, and so to find threads that have a row with user1 in
> the through table, and also have user2 in the through table:
>
> Thread.objects.filter(user=user1).filter(user=user2)
>
> You started by asking how to find threads which had user1 and user2,
> but no other users. This is more complex, you need to count the number
> of users in the thread without filtering the users first! This means
> two queries, or a sub query:
>
> threads_with_both = Thread.objects.filter(user=user1).filter(user=user2)
> threads_with_only_both = Thread.objects.filter(
> pk__in=threads_with_both).annotate(
> num_users=Count('user')).filter(
> num_users=2)
>
> >
> > And please tell me what's the difference between | and &? Because I had
> very
> > different results with these two.
>
> | is OR
> & is AND
>
> So to find threads with either user1 or user2 (or both):
>
> Thread.objects.filter(Q(user=user1) | Q(user=user2))
>
> To find hidden threads with user1
>
> Thread.objects.filter(Q(user=user1) & Q(is_hidden=True))
>
> which is the same as
>
> Thread.objects.filter(user=user1, is_hidden=True)
>
>
> Cheers
>
> Tom
>
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