How to create a Django queryset from a list of values

Question

I am given a list like so

a = [1, 2, 3, 4]

This list matches the id's stored in a django model we will call Books.

I am trying to return a queryset of Books with the id's in the list. I am thinking of using Q objects and grab them by doing something like this

Books.objects.filter(Q(id = 1) | Q(id=2) | Q(id=3) | Q(id=4))

Now i would just use the index like this:

Books.objects.filter(Q(id = a[0]) | Q(id=a[1]) | Q(id=a[2]) | Q(id=a[3]))

BUT, These list will be varying in length up to around 30 and hardcoding the index wont work, and hardcoding the query won't satisfy the varying length of lists.

Is it possible to go this route? And if so, how can I accomplish returning the books with the id's that match the list?

willeM_ Van Onsem · Accepted Answer · 2019-07-23 18:12:35Z

10

You should use the __in lookup [Django-doc] here:

Books.objects.filter(pk__in=a)

As the documentation says:

In a given iterable; often a list, tuple, or queryset. It’s not a common use case, but strings (being iterables) are accepted.

answered Jul 23, 2019 at 18:12

willeM_ Van Onsem

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8 Comments

Mfreeman Over a year ago

If you could, how could i accomplish this with a list of strings instead of integers?

willeM_ Van Onsem Over a year ago

@Mfreeman: the same way, just a = ['foo', 'bar'], for example Books.objects.filter(title__in=a).

Mfreeman Over a year ago

Are you sure? in the docs it shows Entry.objects.filter(id__in=[1, 3, 4]) Entry.objects.filter(headline__in='abc') as equivalents to these sql commands SELECT ... WHERE id IN (1, 3, 4); SELECT ... WHERE headline IN ('a', 'b', 'c');

willeM_ Van Onsem Over a year ago

@Mfreeman: I'm quite sure, are you sure that the list contains the exact same string (no leading/trailing spaces, etc.)? Did you pass it as a list of strings, not a single string.

willeM_ Van Onsem Over a year ago

@MFreeman: well you can check the query it generates with print(Books.objects.filter(...).query). But I'm actually really surprised.

|

nigel222 · Accepted Answer · 2019-07-24 08:40:32Z

2

For a simple case, use __in lookup as per the answer above

However, Q objects connected with | is not magic syntax, so it ought to be possible to build chains of them using loops, like

qlst = []
for x in some_iterable
    qel = Q( something based on x)
    qlst.append(qel)
if len(qlst) == 0:
    # oops -- must handle somehow, or code below will crash
filter = qlst[0]
for qel in qlst[1:]:
    filter = filter | qel

queryset = Books.objects.filter( filter)

Caveat: I've never actually needed to try this.

answered Jul 24, 2019 at 8:40

nigel222

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