从Django queryset仅获取一个相关对象

Question

I'm trying to optimize a feed-like query in Django where I get the unique items commented by users I follow.

        queryset_comments = Item.objects.distinct() \
        .prefetch_related('comments', 'comments__user') \
        .filter(comments__user__in=self.request.user.following.all()) \
        .prefetch_related(
            Prefetch('comments', queryset=Comment.objects.filter(
                user__in=self.request.user.following.all()
            ).order_by('-created_on'), to_attr='activity')
        ) \
        .all()

This gives me a related activity object with a list of all the comments made by user I follow, with the most recent comment first. Now I only need the first one, so I loop other the queryset and set the values I need:

        for item in queryset_comments:
            item.feed_user = item.activity[0].user
            item.feed_date = item.activity[0].created_on
            item.feed_activity = 'commented'

But this loop is taking more than 2 seconds with only about 500 items... I've tried to prefetch with slicing (by adding [0] or .first() to the Prefetch queryset, but both methods are unsupported by Django ORM.

Any suggestion to make this a bit faster ?

Answer 1

There was a simliar question here that made me come to the same conclusion.

I would suggest to not use prefetch_related , but to finish working your queryset until its ready to execute the database query. Maybe even paginated, so you only have 20 or 50 objects.

Then make a list of all Item.pk values and fetch the related "one comment per Item" yourself (using Max('created_on') ).

Same thing as prefetch_related but more control.