Python 如何向 Django QuerySet 添加附加列
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How to add additional column to Django QuerySet
提问by dev9
I have a QuerySet with Books and I would like to add a scorefield to every Book result.
我有一个带有 Books 的 QuerySet，我想score为每个 Book 结果添加一个字段。
qs = Book.objects.all()
In raw SQL I would write:
在原始 SQL 中，我会写：
SELECT
*,
(
(SELECT COUNT(*) FROM votes WHERE value=1 AND book=b.id) -
(SELECT COUNT(*) FROM votes WHERE value=-1 AND book=b.id)
) AS score
FROM
Book b;
How can I achieve it in Django? I tried annotate(), but it seems it's not meant for this kind of stuff.
我怎样才能在 Django 中实现它？我试过了annotate()，但它似乎不适合这种东西。
采纳答案by ambi
If votes possible values are only 1 and -1 you can just sum them using mentioned annotate: Book.objects.annotate(score=Sum('votes__value')).
如果投票可能的值只有 1 和 -1，您可以使用提到的annotate 将它们相加：Book.objects.annotate(score=Sum('votes__value'))。
If there is more possible values you can filter annotation by adding .filter(votes__value__range=(1,1))to the above query.
如果有更多可能的值，您可以通过添加.filter(votes__value__range=(1,1))到上述查询来过滤注释。
If it's more complex you would have to use extrawith select.
如果它更复杂，则必须使用extrawith select。
