CODEWITHSUNDEEP
pythondjangoindexingdjango-queryset
Deniz Dogan
Deniz Dogan

Reputation: 26227

Get the index of an element in a queryset

I have a QuerySet, let's call it qs, which is ordered by some attribute which is irrelevant to this problem. Then I have an object, let's call it obj. Now I'd like to know at what index obj has in qs, as efficiently as possible. I know that I could use .index() from Python or possibly loop through qs comparing each object to obj, but what is the best way to go about doing this? I'm looking for high performance and that's my only criteria.

Using Python 2.6.2 with Django 1.0.2 on Windows.

Upvotes: 53

Views: 66530

Answers (6)

drdaeman
drdaeman

Reputation: 11481

If you're already iterating over the queryset and just want to know the index of the element you're currently on, the compact and probably the most efficient solution is:

for index, item in enumerate(your_queryset):
    ...

However, don't use this if you have a queryset and an object obtained by some unrelated means, and want to learn the position of this object in the queryset (if it's even there).

Upvotes: 69

Peter Brooks
Peter Brooks

Reputation: 1200

It's possible for a simple pythonic way to query the index of an element in a queryset:

(*qs,).index(instance)

This answer will unpack the queryset into a list, then use the inbuilt Python index function to determine it's position.

Upvotes: 11

Jiaaro
Jiaaro

Reputation: 76928

You can do this using queryset.extra(…) and some raw SQL like so:

queryset = queryset.order_by("id")
record500 = queryset[500]

numbered_qs = queryset.extra(select={
    'queryset_row_number': 'ROW_NUMBER() OVER (ORDER BY "id")'
})

from django.db import connection
cursor = connection.cursor()
cursor.execute(
    "WITH OrderedQueryset AS (" + str(numbered_qs.query) + ") "
    "SELECT queryset_row_number FROM OrderedQueryset WHERE id = %s",
    [record500.id]
    )
index = cursor.fetchall()[0][0]

index == 501 # because row_number() is 1 indexed not 0 indexed

Upvotes: 5

Richard
Richard

Reputation: 1863

If you just want to know where you object sits amongst all others (e.g. when determining rank), you can do it quickly by counting the objects before you:

    index = MyModel.objects.filter(sortField__lt = myObject.sortField).count()

Upvotes: 58

Vinay Sajip
Vinay Sajip

Reputation: 99415

Assuming for the purpose of illustration that your models are standard with a primary key id, then evaluating

list(qs.values_list('id', flat=True)).index(obj.id)

will find the index of obj in qs. While the use of list evaluates the queryset, it evaluates not the original queryset but a derived queryset. This evaluation runs a SQL query to get the id fields only, not wasting time fetching other fields.

Upvotes: 25

rob
rob

Reputation: 37644

QuerySets in Django are actually generators, not lists (for further details, see Django documentation on QuerySets).
As such, there is no shortcut to get the index of an element, and I think a plain iteration is the best way to do it.

For starter, I would implement your requirement in the simplest way possible (like iterating); if you really have performance issues, then I would use some different approach, like building a queryset with a smaller amount of fields, or whatever.
In any case, the idea is to leave such tricks as late as possible, when you definitely knows you need them.
Update: You may want to use directly some SQL statement to get the rownumber (something lie . However, Django's ORM does not support this natively and you have to use a raw SQL query (see documentation). I think this could be the best option, but again - only if you really see a real performance issue.

Upvotes: 25

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