Django:使用 Django ORM 实现 JOIN? [英] Django: implementing JOIN using Django ORM?
问题描述
我有一个用 Django 构建的 Q&A 类型的网站,具有以下模型:
I have a Q&A type of site built in Django with the following models:
class Question(models.Model):
title = models.CharField(max_length=70)
details = models.TextField()
class Answer(models.Model):
question_id = IntegerField()
details = models.TextField()
我需要显示一个特定的问题及其答案.通常我需要 2 个查询才能做到这一点:
I need to display a specific question together with its answers. Normally I'd need 2 queries to do that:
Question.objects.get(id=1)
Answer.objects.get(question_id=1)[:10]
我希望使用一个查询来检索所有内容.在 MySQL 中它会是:
I'm hoping to retrieve everything using one query. In MySQL it'd be:
SELECT *
FROM Question JOIN Answer ON Question.id=Answer.question_id
WHERE Question.id=1
LIMIT 10
无论如何我可以通过 Django 的 ORM 做到这一点吗?extra()
在这种情况下会有帮助吗?
Is there anyway I could do this through Django's ORM?
Would extra()
help in this case?
推荐答案
这正是 select_related() 确实如此.唯一的问题是你必须从答案模型开始,而不是问题,但是结果是一样的:
This is exactly what select_related() does. The only gotcha is that you have to start with the Answer model, rather than Question, but the result is the same:
answers = Answer.objects.filter(question_id=1).select_related()
现在每个答案对象都有一个预取的问题"属性,并且访问它不会再次命中数据库.
Now each answer object has a pre-fetched 'question' attribute, and accessing it won't hit the db again.
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