Making a queryset, but with filters that come from a list

Question

I'm making a flashcard-like web app using django. I have a HTML form with a checklist so the user can select the topics they want to review. The HTML form will pass the selected topics as a list to my python script.

chosenTopics = ["topic1", "topic2", "topic3"]

In order to retrieve objects from the database that are from "topic1", I used the following:

cardList = card.objects.all().filter(cardCategory=chosenTopics[0])

I would get cardList as a queryset. If I wanted objects from topic1 and topic2, I can do it like so:

cardList1 = card.objects.all().filter(cardCategory=chosenTopics[0])
cardList2 = card.objects.all().filter(cardCategory=chosenTopics[1])
cardList = cardList1 | cardList2

However, I'm stuck on how I could get a final queryset from all the topics in my chosenTopics list. There are 8 different topics on my HTML form, how can I get a final queryset from any combination of the 8?

Any help is greatly appreciated!

UPDATE:
I messed around and managed to do it with the filter like this:

card.objects.all().filter(cardCategory__in=chosenTopics)

However, I have no idea why that works. What do the 2 underscores do, and what other operations can I do with them?

Answer 1

Hi,
Good job solving it on your own.
This syntax seems totally specific to the Django QuerySet object.
I found the documentation of filtering conditions here:
https://docs.djangoproject.com/en/2.1/ref/models/querysets/#field-lookups
Scroll down to the 'in' section.
This is producing a SQL query with a condition something like:

SELECT * FROM... WHERE cardCategory IN chosenTopics