Custom ordering by calculation in Django - django

Any solutions for custom calculation sorting in Django? I want to create a view that shows the Top Posts in my Blog. The ranking will be calculated by Post's attributes. Let's just say I have 3 IntegerFields called x, y, and z, and the ranking calculation will be x * y / z.
Any ideas? I would like to do Top Post ever, and also other variations filtered by time such as last 24 hours, 7 days, 1 month, etc.
Thanks!

You can use extra to retrieve extra calculated column(s) and sort by it:
MyModel.objects.filter(post_date__lt=#date#)
.extra(select={'custom_order': "x*y/z"}).order_by('custom_order')
The problem with this approach is that you're writing sql so it is not always portable across databases (although, for the example you supplied, this problem is avoided because it's a simple calculation)
Otherwise, you can do the sorting with pure python:
sorted_models = sorted(MyModel.objects.filter(post_date__lt=#date#)
, key=lambda my_model:my_model.x*my_model.y/my_model.z))

The extra() queryset method should allow you to do this. See the docs

As you can't order querysets by methods and properties in django you have to do the sorting in python.
Consider turning your calculated field into a property on your model and then you can do this in your view:
sorted_posts = sorted(Post.objects.all(), key=lambda post: post.calculated_field )
Finally you can pass sorted_posts to your list-template.

Related

complex query in django view from different models

I have three models, Which has some common but not exact fields, from a single view ex. home I am retrieving like this
interviews = Interviews.objects.all().order_by('pub_date')[:3]
publications = Publications.objects.all().order_by('pub_date')[:3]
published = Published.objects.all().order_by('pub_date')[:3]
From home view I want them to show in template in such order that all the latest/New entries associated with these models will be in top.
like if interview entry 10 is the most recent entry in these all models, then it will be first, then if the published one is second recent it will be second ... etc .etc
Can any one tell me how to do it?
Depending on your other requirements it may be a good idea to make them into sublcasses of a common superclass - containing the common elements.
It's hard to say if it's valid, but if you do you can query the different types of objects separately or together by calling
SuperClass.objects.all().order_by('-pub_date')[:9]
which will render the 9 first objects regerdless of what sublcass they are. Of course assuming the superclass is named SuperClass. Of course this does not insure that there are 3 of each model.
Another simple way of solving it - although admittedly not using a query would simply be to sort the lists.
entries = sorted(list(interviews) + list(publications) + list(published), key=lambda x: x.pub_date, reverse=True)
should work - basically turning them into lists and sorting them.
One possible way is using lambda to sort data, but costs are hiher since you will doing this to python, not DBMS...
lst = []
lst.extend(list(Interviews.objects.order_by('pub_date')[:10]))
lst.extend(list(Publications.objects.order_by('pub_date')[:10]))
lst.extend(list(Published.objects.order_by('pub_date')[:10]))
# take 10 records for each, since you could not know how many records will be picked from which table
# now order them...
lst.sort(lambda x, y: cmp(x.pub_date, y.pub_date))
# and reverse the order, so newset is the first...
lst.reverse()
this will give you a list of objects ordered py pub_date, so you can slice the final list to get any number of records you want...
lst = lst[:10]

Django: Order by number of comments issue

I'm trying to order a list of items in django by the number of comments they have. However, there seems to be an issue in that the Count function doesn't take into account the fact that django comments also uses a content_type_id to discern between comments for different objects!
This gives me a slight problem in that the comment counts for all objects are wrong using the standard methods; is there a 'nice' fix or do I need to drop back to raw sql?
Code to try and ge the correct ordering:
app_list = App.objects.filter(published=True)
.annotate(num_comments=Count('comments'))
.order_by('-num_comments')
Sample output from the query (note no mention of the content type id):
SELECT "apps_app"."id", "apps_app"."name",
"apps_app"."description","apps_app"."author_name", "apps_app"."site_url",
"apps_app"."source_url", "apps_app"."date_added", "apps_app"."date_modified",
"apps_app"."published", "apps_app"."published_email_sent", "apps_app"."created_by_id",
"apps_app"."rating_votes", "apps_app"."rating_score", COUNT("django_comments"."id") AS
"num_comments" FROM "apps_app" LEFT OUTER JOIN "django_comments" ON ("apps_app"."id" =
"django_comments"."object_pk") WHERE "apps_app"."published" = 1 GROUP BY
"apps_app"."id", "apps_app"."name", "apps_app"."description", "apps_app"."author_name",
"apps_app"."site_url", "apps_app"."source_url", "apps_app"."date_added",
"apps_app"."date_modified", "apps_app"."published", "apps_app"."published_email_sent",
"apps_app"."created_by_id", "apps_app"."rating_votes", "apps_app"."rating_score" ORDER
BY num_comments DESC LIMIT 4
Think I found the answer: Django Snippet

Django annotation with nested filter

Is it possible to filter within an annotation?
In my mind something like this (which doesn't actually work)
Student.objects.all().annotate(Count('attendance').filter(type="Excused"))
The resultant table would have every student with the number of excused absences. Looking through documentation filters can only be before or after the annotation which would not yield the desired results.
A workaround is this
for student in Student.objects.all():
student.num_excused_absence = Attendance.objects.filter(student=student, type="Excused").count()
This works but does many queries, in a real application this can get impractically long. I think this type of statement is possible in SQL but would prefer to stay with ORM if possible. I even tried making two separate queries (one for all students, another to get the total) and combined them with |. The combination changed the total :(
Some thoughts after reading answers and comments
I solved the attendance problem using extra sql here.
Timmy's blog post was useful. My answer is based off of it.
hash1baby's answer works but seems equally complex as sql. It also requires executing sql then adding the result in a for loop. This is bad for me because I'm stacking lots of these filtering queries together. My solution builds up a big queryset with lots of filters and extra and executes it all at once.
If performance is no issue - I suggest the for loop work around. It's by far the easiest to understand.
As of Django 1.8 you can do this directly in the ORM:
students = Student.objects.all().annotate(num_excused_absences=models.Sum(
models.Case(
models.When(absence__type='Excused', then=1),
default=0,
output_field=models.IntegerField()
)))
Answer adapted from another SO question on the same topic
I haven't tested the sample above but did accomplish something similar in my own app.
You are correct - django does not allow you to filter the related objects being counted, without also applying the filter to the primary objects, and therefore excluding those primary objects with a no related objects after filtering.
But, in a bit of abstraction leakage, you can count groups by using a values query.
So, I collect the absences in a dictionary, and use that in a loop. Something like this:
# a query for students
students = Students.objects.all()
# a query to count the student attendances, grouped by type.
attendance_counts = Attendence(student__in=students).values('student', 'type').annotate(abs=Count('pk'))
# regroup that into a dictionary {student -> { type -> count }}
from itertools import groupby
attendance_s_t = dict((s, (dict(t, c) for (s, t, c) in g)) for s, g in groupby(attendance_counts, lambda (s, t, c): s))
# then use them efficiently:
for student in students:
student.absences = attendance_s_t.get(student.pk, {}).get('Excused', 0)
Maybe this will work for you:
excused = Student.objects.filter(attendance__type='Excused').annotate(abs=Count('attendance'))
You need to filter the Students you're looking for first to just those with excused absences and then annotate the count of them.
Here's a link to the Django Aggregation Docs where it discusses filtering order.

django queryset datetime values substraction

I have model with following field.
date = models.DateTimeField(auto_now_add=True)
When querying such model i`d like to have additional column that would keep difference between current date and previous one. So for 10 rows it would have 9 values, first one would be None. Are there any ways of achieving this with querysets? or maybe i should mess around and create additional list that would hold such differences before i pass queryset to template?
Please advise.
Create a property on the model that calls get_ {next,previous}_ by_*() and returns the timedelta. For advanced functionality, implement caching.

django aggregate aggregated fields?

I have a model called Item, with m2m relation to User ("owner").
For each item, I need to count users who own it. That's easy enough with annotate()
But then I need to calculate ratio between owners of specific gender and total owner count for each item. For example if, 2 males own the item out of 5 users, the ratio is 0.4.
What's the best way to do that?
To do this with the ORM, you need conditional aggregates, which aren't supported in Django. http://www.voteruniverse.com/Members/jlantz/blog/conditional-aggregates-in-django proposes a hacky solution that might work.
If you don't need to sort by the ratio, then you can make two calls to annotate, and then compute the ratio in Python. Something like:
items = Item.objects.annotate(ucount=Count('users')).annotate(ccount=CountIf(<condition>))
for item in items:
item.ratio = item.ucount / item.ccount
If you don't want to do that, I'd recommend using the extra() method and some custom sql to get the extra info you want. Documentation for that method is on the Django Queryset API documentation page.
Just on top of my head, something like the following could work. Iterate on it to get your perfect solution if you wish:
items = Item.objects.annotate(Count('users'))
for item in items:
total = item.users__count
num_males = item.users.filter(gender='M').count()
num_females = item.users.filter(gender='F').count()