问题

我想知道在我需要检索外键模型对象时，最佳的流程是什么。

<br/>在views.py中使用**prefetch_related**是否是一个选择:
```python
queryset = queryset.prefetch_related('book_set')
context={objects:queryset}

或者直接在模板中使用model_set:

{% for attach in objects.book_set.all %}

哪个更好，哪一个对性能更好？

<details>
<summary>英文:</summary>

I wonder what is the best flow when I need to retrieve foreign key model objects.
========
 &lt;br/&gt;is it to use **prefetch_related** in views.py like:

queryset = queryset.prefetch_related('book_set')
context={objects:queryset}

or to use directly in the template by **model_set**:

{% for attach in objects.book_set.all %}

what is better and which one is good for performance?

</details>


# 答案1
**得分**: 1

在Django中，[prefetch_related][1]用于提前选择所有父对象的所有相关对象，主要与`ManyToMany`字段和反向关系一起使用。

而[model_set][2]用于获取单个父对象的所有相关对象，这将明显增加数据库的访问次数。

因此，最好使用`prefetch_related`，因为它将显著减少数据库的访问次数。

如果您不进行反向关系并且具有外键关系，那么首选使用[select_related][3]。

就在模板或视图中编写查询而言，我建议始终在视图中执行所有查询，因为模板会增加额外的开销。

[1]: https://docs.djangoproject.com/en/4.2/ref/models/querysets/#prefetch-related
[2]: https://docs.djangoproject.com/en/4.0/topics/db/queries/#related-objects
[3]: https://docs.djangoproject.com/en/4.2/ref/models/querysets/#select-related

<details>
<summary>英文:</summary>

In Django [prefetch_related][1] is used to select all related objects for all parent objects in advance mostly used with `ManyToMany` field and a reverse relationship. 

While as [model_set][2] is used to get all related objects of a single parent object which will definitely have more database hits.

Hence it is better to do `prefetch_related` as it will reduced the database hits significantly.

If you are not doing reverse relationship and have foreign key relation then [select_related][3] is prefered.

As far as writting queries in template or views is better. I would suggest always do all queries in the view as templates add extra level of overhead.


  [1]: https://docs.djangoproject.com/en/4.2/ref/models/querysets/#prefetch-related
  [2]: https://docs.djangoproject.com/en/4.0/topics/db/queries/#related-objects
  [3]: https://docs.djangoproject.com/en/4.2/ref/models/querysets/#select-related

</details>



# 答案2
**得分**: 0

最佳方法取决于您的具体用例以及数据库的内容和大小。但一般来说，在``views.py``中使用`prefetch_related`可以通过减少数据库查询来提供更好的性能。

以下是一个示例，用于澄清这两种方法：

1. 在views.py中使用`prefetch_related()`：

```python
# views.py

queryset = Book.objects.all().prefetch_related('author')
context = {'books': queryset}

<!-- template.html -->

{% for book in books %}
    {{ book.title }} by {{ book.author.name }}
{% endfor %}

在这种方法中，prefetch_related('author')会在单个查询中获取所有相关的作者，从而减少了数据库查询次数。

2. 在模板中使用model_set：

<!-- template.html -->

{% for book in books %}
    {{ book.title }} by
    {% for author in book.author_set.all %}
        {{ author.name }}
    {% endfor %}
{% endfor %}

而在第二种方法中，每次迭代book.author_set.all都会访问数据库以检索相关的作者对象。这可能会导致多次数据库查询，并且可能会出现各种问题，特别是如果您有大量的书籍。

# views.py

queryset = Book.objects.all().prefetch_related('author')
context = {'books': queryset}

<!-- template.html -->

{% for book in books %}
    {{ book.title }} by {{ book.author.name }}
{% endfor %}

<!-- template.html -->

{% for book in books %}
    {{ book.title }} by
    {% for author in book.author_set.all %}
        {{ author.name }}
    {% endfor %}
{% endfor %}