Django：使用Django ORM实现JOIN？ [英] Django: implementing JOIN using Django ORM?

问题描述

我有一个类似Django的Q&A类型的站点，其中包含以下模型：

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()

我需要一起显示一个特定的问题与答案。通常我需要两个查询：

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。在MySQL中，它将是：

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

有没有办法这通过Django的ORM？
在这种情况下， extra（）帮助

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（）。唯一的问题是
你必须从答案模型开始，而不是问题，但
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先提取的问题属性，而
访问它不会再次击中db。

Now each answer object has a pre-fetched 'question' attribute, and accessing it won't hit the db again.

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