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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