Python 仅返回serializer-Django中相关模型的最后一个条目

我有两个Django模型，讨论和发布设置如下：

讨论模式：

class Discussion(models.Model):
    Discussion_ID = models.AutoField(primary_key=True)
    Topic = models.CharField(max_length=50)
    Group = models.ForeignKey(Group, related_name="Group_Discussions")

    class Meta:
        db_table = "Discussions"        

后模型：

class Post(models.Model):
    Post_ID = models.AutoField(primary_key=True)
    Date_Posted = models.DateTimeField(auto_now_add=True)
    Discussion = models.ForeignKey(Discussion, db_column="Discussion_ID", related_name="Posts")
    User = models.ForeignKey(User, related_name="User_Posts")
    Message = models.TextField()

    class Meta:
        db_table = "Posts"

我想序列化所有讨论的列表，并且在序列化程序中，只包含每个讨论的最新帖子。到目前为止，我的序列化程序如下所示：

class DiscussionSerializer(serializers.ModelSerializer):
    last_post = serializers.SerializerMethodField()
    post_count = serializers.SerializerMethodField()

    class Meta:
        model = Discussion
        fields = ('Discussion_ID', 'Topic', 'last_post', 'post_count')

    def get_last_post(self, obj):
        return Post.objects.raw("SELECT * FROM Posts WHERE Discussion_ID = %s ORDER BY Post_ID DESC LIMIT 1" % obj.Discussion_ID)

    def get_post_count(self, obj):
        return obj.Posts.count()

{ 
    "Discussion_ID": 1, 
    "Topic": "Some topic",
    "last_post": { 
        "Post_ID": 1,
        "Date_Posted": "...",
        "Discussion": 1,
        "User": 1,
        "Message": "This is a message"
    },
    "post_count": 10
}

不幸的是，这将返回以下错误，我不太确定还要尝试什么：

<RawQuerySet: SELECT * FROM Posts WHERE Discussion_ID = 1 ORDER BY Post_ID DESC LIMIT 1> is not JSON serializable

---

我通过以下方式实现了这一目标：

def get_last_post(self, obj):
    try:
        post = obj.Posts.latest('Date_Posted')
        serializer = PostSerializer(post)
        return serializer.data
    except Exception, ex:
        return None

---

虽然感觉有点黑客，但我仍然对其他解决方案持开放态度。

要改进您的解决方案，您可能需要将此代码移动到viewset方法，并处理一些其他问题，如权限、为

帖子应用过滤器（如果有）和“未找到”错误。例如：

class PostViewSet(viewsets.GenericViewSet, mixins.RetrieveModelMixin):

    #...

    def retrieve_last():

        queryset = self.filter_queryset(self.get_queryset())

        try:
            obj =  queryset.latest('Date_Posted')
        except queryset.model.DoesNotExist:
            raise Http404('No %s matches the given query.' \
                          % queryset.model._meta.object_name)

        self.check_object_permissions(self.request, obj)

        serializer = self.get_serializer(obj)

        return Response(serializer.data)

顺便说一下，这是受DRF的
ModelViewSet
启发的