Django：如何为模型实例预取相关信息。也许通过包装在一个查询集中？ [英] Django: How to prefetch related for a model instance. Perhaps by wrapping in a queryset?

问题描述

我使用Django rest框架，并且在模型关系中具有不错的嵌套。

I use Django rest framework and I have decent nesting in my model relations.

我正在优化查询。我的许多函数都使用或操纵一个模型实例，并且它通常位于数据流的更下游，因此我需要进行一些预取。一个典型的例子是DRF串行器。这是一个示例。

I'm working on optimizing my queries. Many of my functions consume or manipulate a single instance of model and it's often further downstream in the data flow that it turns out I need some prefetching. One classic instance of this is with DRF serializers. Here's an example.

@api_view(['GET'])
def fetch_user_profile(request):
    profile = request.user.profile # has many nested relationships

    return Response(UserProfileSerializer(profile).data, status=status.HTTP_200_OK) # this ends up taking many queries

我见过一些解决方案，建议在上下文中使用 prefetch_related 传递查询集尽管我无法完全了解它的工作方式（我只发现了几个地方对此进行了部分讨论，但可能会对此提出其他问题）。

I've seen some solutions suggesting passing queryset with prefetch_related in the context although I haven't been able to get a full picture on how that would work (I've only found a couple of places that discuss it partially, might open a whole other question on that).

一个简单的解决方法（可能会超出序列化程序）：如果我可以使用包装函数将实例包装到查询集中，然后调用prefetch并将其传递给序列化程序

类似的东西

def queryset_wrap(instance):
    return type(instance).objects.filter(id=instance.id)

我想知道是否有更好的方法可以做到这一点。 / strong>

I'm wondering if there's a better way to do this.

推荐答案

我自己尚未使用此功能，但我相信它是在Django 1.10中引入的prefetch_related_objects（）函数

I haven't yet used this myself, but I believe that it is the prefetch_related_objects() function that you are looking for, introduced in Django 1.10

从文档中：

预取给定在可迭代的模型实例上进行查找。这在接收模型实例列表而不是QuerySet的代码中很有用；例如，当从缓存中获取模型或手动实例化模型时。

Prefetches the given lookups on an iterable of model instances. This is useful in code that receives a list of model instances as opposed to a QuerySet; for example, when fetching models from a cache or instantiating them manually.

传递可迭代的模型实例（必须全部属于同一类）以及要预取的查找或Prefetch对象对于。例如：

Pass an iterable of model instances (must all be of the same class) and the lookups or Prefetch objects you want to prefetch for. For example:

>>> from django.db.models import prefetch_related_objects
>>> restaurants = fetch_top_restaurants_from_cache()  # A list of Restaurants
>>> prefetch_related_objects(restaurants, 'pizzas__toppings')

由于它需要迭代，因此可以将实例包装在列表中：

Since it takes an iterable, you can wrap your instance in a list:

prefetch_related_objects([profile], "relevant_attribute_1", "relevant_attribute_2")

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