Django REST框架序列化程序:显示反向关系的最新对象

Question

ListAPIView(下面的代码)的默认行为是序列化所有Report对象和每个Report对象的嵌套Log对象.如果我只希望每个报告显示最新的Log对象怎么办？我该怎么做呢？

# models.py
class Log(models.Model):
    # ...
    report = models.ForeignKey(Report)
    timestamp = models.DateTimeField(default=datetime.datetime.now)

class Report(models.Model):
    code = models.CharField(max_length=32, unique=True)
    description = models.TextField()


# serializers.py
class LogSerializer(serializers.ModelSerializer):

    class Meta:
        model = Log

class ReportSerializer(serializers.ModelSerializer):
    log_set = LogSerializer(many=True, read_only=True)

    class Meta:
        model = Report
        fields = ('code', 'description', 'log_set')


# views.py
class ReportListView(generics.ListAPIView):

    queryset = Report.objects.all()
    serializer_class = ReportSerializer

我知道我可以通过使用SerializerMethodField来实现这一点,但这可能是一个可能很昂贵的操作,因为会有一个额外的SQL查询来检索每个Report对象的相应Log对象.

class ReportSerializer(serializers.ModelSerializer):
    latest_log = serializers.SerializerMethodField()

    class Meta:
        model = Report

    def get_latest_log(self, obj):
        try:
            latest_log = Log.objects.filter(report_id=obj.id).latest('timestamp')
        except Log.DoesNotExist:
            latest_log = None
        return latest_log

如果我有1000个报表对象,如果我想要全部渲染它们,将会有1000个额外的查询.除了使用分页之外,我如何避免这些额外的查询？有人能指出我正确的方向吗？谢谢!

编辑:关于可能的重复标签,马克提供的链接并没有完全清除我的图片.Todor的回答更清楚.

Answer 1

你需要以某种方式注释latest_log的ReportQuerySet,因此它可以通过串行未做任何额外的查询中使用.

实现这一目标的最简单方法是prefetching所有的logs每report.这种方法的缺点是,你在内存中加载所有logs每report每页.如果有人report得到5-10-15之类的东西,那就不那么糟糕了logs.这意味着对于一个50的页面reports,你将加​​载50*10 = 500 logs,这不是什么大问题.如果有更多logsper report(比如说100)那么你需要对其进行额外的过滤queryset.

这是一些示例代码:

1. 预取logs.

   # views.py
   class ReportListView(generics.ListAPIView):
   
       queryset = Report.objects.all()\
                   .prefetch_related(Prefetch('log_set', 
                       queryset=Log.objects.all().order_by('-timestamp'),
                       to_attr='latest_logs'
                   ))
      serializer_class = ReportSerializer
   

2. 创建一个帮助方法,以便轻松访问latest_log

   class Report(models.Model):
       #...
   
       @property
       def latest_log(self):
           if hasattr(self, 'latest_logs') and len(self.latest_logs) > 0:
               return self.latest_logs[0]
           #you can eventually implement some fallback logic here 
           #to get the latest log with a query if there is no cached latest_logs 
           return None
   

3. 最后,序列化程序只使用该属性

   class ReportSerializer(serializers.ModelSerializer):
       latest_log = serializers.LogSerializer()
   
       class Meta:
           model = Report
   

更高级过滤的示例logs可以是这样的:

Report.objects.all().prefetch_related(Prefetch('log_set', queryset=Log.objects.all().extra(where=[
    "`myapp_log`.`timestamp` = (\
        SELECT max(timestamp) \
        FROM `myapp_log` l2 \
        WHERE l2.report == `myapp_log`.`report`\
    )"]
    ), to_attr='latest_logs'
))