Django REST Framework Serializers: Display the latest object of a reverse relationship

Question

The default behavior of the ListAPIView (code below) is to serialize all Report objects and the nested Log objects per Report object. What if I only want the latest Log object to be displayed per Report? How do I go about doing that?

# 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

I know I can do this by using a SerializerMethodField, but this can be a potentially expensive operation, since there will be an extra SQL query to retrieve the appropriate Log object for each Report object.

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

If I have 1000 report objects, there will be 1000 extra queries if I want to render them all. How do I avoid those extra queries besides using pagination? Can anyone point me to the right direction? Thanks!

EDIT: Regarding the possible duplicate tag, the link alone provided by Mark did not completely clear up the picture for me. Todor's answer was more clear.

Answer 1

You need to somehow annotate the latest_log in the ReportQuerySet, so it can be used by the serializer without making any extra queries.

The simplest way to achieve this is by prefetching all the logs per report. The drawback of this approach is that you load in memory all the logs per report per page. Which is not so bad if one report got something like 5-10-15 logs. This will mean that for a page with 50 reports, you are gonna load 50*10=500 logs which is not a big deal. If there are more logs per report (lets say 100) then you need to make additional filtering of the queryset.

Here is some example code:

1. Prefetch the 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. Create a helper method for easy access the 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. Finally the serializer just use the property

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

An example of a more advanced filtering of the logs can be something like this:

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'
))

Answer 2

You can use select related argument. It will hit the database only once using JOIN.

class ReportListView(generics.ListAPIView):

    queryset = Report.objects.select_related('log');
    serializer_class = ReportSerializer