是否过滤 Django Rest Framework (DRF) 中的代码示例优化问题答案

【问题标题】：Filtering code example optimization issue in Django Rest Framework (DRF) or Not是否过滤 Django Rest Framework (DRF) 中的代码示例优化问题
【发布时间】：2020-08-02 14:45:52
【问题描述】：

最近我在 DRF 文档中提出了以下代码，用于过滤 APIListView 中的查询集，

class PurchaseList(generics.ListAPIView):
    serializer_class = PurchaseSerializer

    def get_queryset(self):
        """
        Optionally restricts the returned purchases to a given user,
        by filtering against a `username` query parameter in the URL.
        """
        queryset = Purchase.objects.all()
        username = self.request.query_params.get('username', None)
        if username is not None:
            queryset = queryset.filter(purchaser__username=username)
        return queryset

看来上面的sn-p代码写成这样会更有效率，

class PurchaseList(generics.ListAPIView):
    serializer_class = PurchaseSerializer

    def get_queryset(self):
        """
        Optionally restricts the returned purchases to a given user,
        by filtering against a `username` query parameter in the URL.
        """
        queryset = None
        username = self.request.query_params.get('username', None)
        if username is not None:
            queryset = queryset.filter(purchaser__username=username)
        else:
            queryset = Purchase.objects.all()
        return queryset

错了吗？还是我想念一些关于 Django ORM 的东西？

【问题讨论】：

标签： django optimization django-rest-framework filtering django-orm

【解决方案1】：

只要不评估查询集，就没有数据库活动。

回答这个问题：不，第二个代码示例实际上效率并不高。

例子：

qs = Model.objects.all()  # no database activity

qs = qs.filter(**some_filter)  # still no activity

list(qs)  # data is being fetched now

qs = qs.filter(**other_filter)  # no new activity

list(qs)  # new activity again

另见django docs。

【讨论】：