Re: Comparing two ManyToMany fields for same entries

I think the filter I used does work to detect related friends, here is
a Pythonic example of the same code, but done differently without
filters:

Person.objects.get(name='Person1').friends.get().friends.all()

Instead of returning back 'Person1', when using filters, this returns
back 'Person0'.

Person.objects.get(name='Sean').friends.get().friends.get(name='Person0')

My next question, which would be better optimized for database hits:

* Brett's example
* using the filter version
* using the Pythonic version, but use select_related(depth=2) option?

I am trying to make sure the database hits are optimized as this will
be queries multiple times per page hit for different models. Also,
which would be faster to make sure the page loads are not burden when
lots of these types of queries go through?

Thanks.

On Oct 28, 1:34 pm, Kevin <kveron...@gmail.com> wrote:
> Thanks.  Is this way good for database optimization though?  I guess I
> could use select_related().  I was hoping there was a QuerySet I could
> run.
>
> I made this one, although I'm not sure if it works correctly:
>
> Person.objects.filter(name='Person1').filter(friends__friends__name='Person0')
> This outputs "Person1", if I interchange Person0 with Person2, it
> outputs no results.  If I use Person1 in both instances, it outputs
> 'Person1'.
> Does this QuerySet return the results I want, or should I use Brett's
> suggestion?
>
> Thanks again.
>
> On Oct 28, 1:21 pm, Brett Epps <Brett.E...@quest.com> wrote:
>
> > Try this:
>
> > for friendof0 in Person0.friends.all():
> >     for friendof1 in Person1.friends.all():
> >         if friendof0 == friendof1:
> >             # Person 0 and Person 1 share a friend.
> >         else:
> >             # They have no shared friends.
>
> > Brett
>
> > On 10/28/11 12:59 PM, "Kevin" <kveron...@gmail.com> wrote:
>
> > >Just thought I'd add another example using Python script:
>
> > >Person0 = Person()
> > >Person1 = Person()
> > >Person2 = Person()
> > >Person0.friends.add(Person2)
> > >Person2.friends.add(Person0)
> > >Person2.friends.add(Person1)
> > >Person1.friends.add(Person2)
>
> > >Now, I would like to do the following, but it seems to fail:
>
> > >Person0.friends.all() in Person1.friends.all().  I would like it to
> > >say if Person0 and Person1 share another friend in common.
>
> > >Person0 and Person2 are friends
> > >Person1 and Person2 are friends
> > >Person0 and Person1 are NOT friends, but share a friend in common.
> > >How does one find out that even though Person0 and Person1 are not
> > >friends, they do share Person2 as a friend.
>
> > >I can use my eye on a Python shell to see that Person2 exists on both
> > >Peson0 and Person1, but how does one make the code see it?
>
> > >Sorry for having to clarify this so much, I'm just not sure that my
> > >last post actually explained it properly.
>
> > >Thanks.
>
> > >On Oct 28, 12:30 pm, Kevin <kveron...@gmail.com> wrote:
> > >> Hello,
>
> > >>   I am building a model which shares a relation with another model
> > >> using a ManyToManyField.  What I need to do, is find out which models
> > >> are on both on two seperate ManyToManyField lists.  Here is a simple
> > >> example of what I am trying to do:
>
> > >> Person:
> > >>   friends=ManyToManyField(self)
>
> > >> To find out this persons direct friends, Person.friends...
> > >> To find out which friends this Person shares in common with another
> > >> Person, ????
>
> > >> Person0:
> > >>   Person1
> > >>   Person6
> > >>   Person3
> > >>   Person8
>
> > >> Person1:
> > >>   Person2
> > >>   Person6
>
> > >> What would be the most optimized QuerySet to find out that both
> > >> Person0 and Person1 are both friends with Person6?
>
> > >> Is there a specific Django app perhaps that can ease develop of this
> > >> type of data relations between objects?
>
> > >> Thanks.
>
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