Re: values() and order_by()
On Wed, Jul 4, 2012 at 1:59 PM, J. Javier Maestro <jjmaes...@ieee.org> wrote: > Hi there! > > Thanks a lot for the reply. However, I don't think your solution would > work in my case, nor it answers the question of whether it can be done > or why the order_by() adds stuff to the GROUP BY clause when used in > conjunction with values(). > > Let's see: > > On Tue, Jul 3, 2012 at 11:08 PM, Bill Freeman <ke1g...@gmail.com> wrote: >> First, I think that you're going about last_visit in the wrong way. >> timezone.now() is evaluated once, when the module is imported, >> rather than when the user visits. DateTimeField's auto_now=True >> option is tailor made for this kind of situation. > > I know about auto_now and, apart from the discussions that can be > found all over the Internet, is a nice feature... unless you update > records (like I do). Then, auto_now does nothing since it only affects > save(). In any case, the default value is actually a leftover from the > initial implementation, right now is not even needed. Bill, Re-reading the update() documentation and the way I implemented the middleware, you are right that auto_now will be the best solution. I will use save() instead of update() and will get that fixed. Thanks a lot for the suggestion! If you have any ideas about the values() and order_by() issue, I will be happy to hear them! Cheers, -- J. Javier Maestro <jjmaes...@ieee.org> -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
Re: values() and order_by()
Hi there!
Thanks a lot for the reply. However, I don't think your solution would
work in my case, nor it answers the question of whether it can be done
or why the order_by() adds stuff to the GROUP BY clause when used in
conjunction with values().
Let's see:
On Tue, Jul 3, 2012 at 11:08 PM, Bill Freeman <ke1g...@gmail.com> wrote:
> First, I think that you're going about last_visit in the wrong way.
> timezone.now() is evaluated once, when the module is imported,
> rather than when the user visits. DateTimeField's auto_now=True
> option is tailor made for this kind of situation.
I know about auto_now and, apart from the discussions that can be
found all over the Internet, is a nice feature... unless you update
records (like I do). Then, auto_now does nothing since it only affects
save(). In any case, the default value is actually a leftover from the
initial implementation, right now is not even needed.
> Next, since you have the user id, I'm going to assume that you have
> an actual user object. In the code below I'll presume that it's
> request.user .
>
> visits_qs = Visit.objects.filter(user=request.user)
> total_visits_count = visits_qs.count()
> latest_visit = visits_qs.order_by('-last_visit')[0]
>
> Yes, this is two database queries, but unless you are seeing a performance
> issue that you've traced to the fact that this is more than one query, this
> code is a heck of a lot more maintainable than trying to make it happen
> in one.
First, this won't work in my implementation. My model stores the
actual count of a given visitor in the count_visits field and the
field is updated every time the visitor visits a user profile (via a
middleware and an F() expression) while your solution stores a full
record every time you want to log a visit to a user profile.
Logging visits your way will eat quite a bit of space in the database.
Also, it's not only about saving space and it's not only about two
database queries. In your implementation, the count() will scan the
whole set of records for the given user. This scan will grow with the
number of records and in this case, taking into account that the whole
purpose is to show a counter for visits to a given user profile... I
think that your proposed workaround does not really fit.
So, I ask again: Why the order_by('colum') when used in conjunction
with values(), adds the column to the GROUP BY?
Thanks,
--
J. Javier Maestro <jjmaes...@ieee.org>
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values() and order_by()
Hi there!
I have the following model:
class Visit(models.Model):
user = models.ForeignKey(User)
visitor = models.ForeignKey(User, related_name="visitor")
last_visit = models.DateTimeField(default=timezone.now())
count_visits = models.IntegerField(default=0)
I would like to make the following SQL query using the Django ORM:
SELECT last_visit, SUM(count_visits) AS total_count_visits
FROM core_visit
WHERE user_id=42
GROUP BY user_id
ORDER BY last_visit DESC
So far, I haven't been able to do it. When I use values('user') and
order_by('-last_visit') the last_visit column is automatically added
to the GROUP BY clause. I've read in the Django doc that this is how
it's supposed to be but I can't seem to understand it. Why do I have
to group the results by whatever I choose as order? What am I missing?
Also, I know I can do a RawQuerySet but unfortunately, I would like to
be able to use a full QuerySet (this query goes into Tastypie and so
far, I haven't been successful in using it with anything but full
QuerySets, not even ValueQuerySets...)
All help and ideas would be greatly appreciated.
Thanks a lot in advanced,
--
J. Javier Maestro <jjmaes...@ieee.org>
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