python - How to get entry using primary key from another model in Django?

Question

I have 2 models, "Listing" and "Watchlist". Watchlist has a primary key that is referencing Listing. For every "Watchlist" object (that has the same user X) I want to get the according "Listing" entry in form of a QuerySet.
I really struggle with this, because I don't know how to incorporate a for-loop into a queryset request.

class Watchlist(models.Model):
    listing = models.ForeignKey(Listing, on_delete=models.CASCADE, related_name="watchlists")
    user = models.ForeignKey(User, on_delete=models.CASCADE)

question from:https://stackoverflow.com/questions/65900542/how-to-get-entry-using-primary-key-from-another-model-in-django

Answer 1

Since listing field has related_name="watchlists", we can filter using related_name like this:

Listing.objects.filter(watchlists__user = X) # X is a user instance.

Note that __ is used for field lookups. __user is for user field lookup.

If listing field does not have any related name, we can filter using model name (Watchlist) with _set to the referencing model.

Listing.objects.filter(Watchlist_set__user = X) # X is a user instance.