Good ways to sort a queryset - Django

Question

what I m trying to do is this    get the 30 Authors with highest score   Author objects order by  -score    30    order the authors by last name     Any suggestions

User · Answer

I just wanted to illustrate that the built-in solutions (SQL-only) are not always the best ones. At first I thought that because Django's QuerySet.objects.order_by method accepts multiple arguments, you could easily chain them:

ordered_authors = Author.objects.order_by('-score', 'last_name')[:30]

But, it does not work as you would expect. Case in point, first is a list of presidents sorted by score (selecting top 5 for easier reading):

>>> auths = Author.objects.order_by('-score')[:5]
>>> for x in auths: print x
... 
James Monroe (487)
Ulysses Simpson (474)
Harry Truman (471)
Benjamin Harrison (467)
Gerald Rudolph (464)

Using Alex Martelli's solution which accurately provides the top 5 people sorted by last_name:

>>> for x in sorted(auths, key=operator.attrgetter('last_name')): print x
... 
Benjamin Harrison (467)
James Monroe (487)
Gerald Rudolph (464)
Ulysses Simpson (474)
Harry Truman (471)

And now the combined order_by call:

>>> myauths = Author.objects.order_by('-score', 'last_name')[:5]
>>> for x in myauths: print x
... 
James Monroe (487)
Ulysses Simpson (474)
Harry Truman (471)
Benjamin Harrison (467)
Gerald Rudolph (464)

As you can see it is the same result as the first one, meaning it doesn't work as you would expect.

User · Answer

Here s a way that allows for ties for the cut-off score   author count   Author objects count   cut off score   Author objects order by  -score   values list  score   min 30  author count   top authors   Author objects filter score  gte cut off score  order by  last name     You may get more than 30 authors in top authors this way and the min 30 author count  is there incase you have fewer than 30 authors

User · Answer

What about  import operator  auths   Author objects order by  -score    30  ordered   sorted auths  key operator attrgetter  last name      In Django 1 4 and newer you can order by providing multiple fields  Reference  https   docs djangoproject com en dev ref models querysets  order-by  order by  fields   By default  results returned by a QuerySet are ordered by the ordering tuple given by the ordering option in the model   s Meta  You can override this on a per-QuerySet basis by using the order by method   Example   ordered authors   Author objects order by  -score    last name    30    The result above will be ordered by score descending  then by last name ascending  The negative sign in front of  -score  indicates descending order  Ascending order is implied

[python] Good ways to sort a queryset? - Django

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