如何在Django DRF中为Post查询集添加author_username自定义属性？

场景 #

作为DRF新手，我现有Post和User两个模型，相关代码如下：

Post模型 #

class Post(models.Model):
    """
        Post model
    """

    title = models.CharField(max_length=250)
    body = models.TextField()
    author = models.ForeignKey(settings.AUTH_USER_MODEL,
                               on_delete=models.CASCADE,
                               related_name='forum_posts')
    parent_post = models.ForeignKey('self',
                                    on_delete=models.CASCADE,
                                    blank=True,
                                    null=True)
    time_stamp = models.DateTimeField(default=timezone.now)

    objects = models.Manager()

Post序列化器 #

class PostSerializer(serializers.ModelSerializer):

    class Meta:
        model = models.Post
        fields = ('id', 'title', 'body', 'parent_post', 'author', 'time_stamp')
        extra_kwargs = {'id': {'read_only': True},
                        'author': {'read_only': True}}

视图类 #

class PostList(generics.ListCreateAPIView):
    permission_classes = [IsAuthenticatedOrReadOnly]
    queryset = models.Post.objects.all()
    serializer_class = serializers.PostSerializer

需求 #

返回Post数据时，为每个对象添加author_username字段，值为对应作者的用户名，且不修改模型添加冗余字段。期望返回的JSON示例：

{
    "id": 1,
    "title": "Example post",
    "body": "Example post",
    "author": 1,
    "parent_post": null,
    "time_stamp": "2022-01-01T00:00:00Z",
    "author_username": "testUser"
}

实现方法 #

方法一：使用SerializerMethodField #

适合需要自定义逻辑的灵活场景：

class PostSerializer(serializers.ModelSerializer):
    # 定义自定义字段
    author_username = serializers.SerializerMethodField()

    # 实现字段取值逻辑，obj为当前Post实例
    def get_author_username(self, obj):
        return obj.author.username

    class Meta:
        model = models.Post
        # 将新字段加入返回字段列表
        fields = ('id', 'title', 'body', 'parent_post', 'author', 'time_stamp', 'author_username')
        extra_kwargs = {'id': {'read_only': True},
                        'author': {'read_only': True}}

方法二：使用CharField+source参数 #

无需额外逻辑时，这种方式更简洁：

class PostSerializer(serializers.ModelSerializer):
    # 通过source直接关联到author的username字段，设置read_only避免提交时被修改
    author_username = serializers.CharField(source='author.username', read_only=True)

    class Meta:
        model = models.Post
        fields = ('id', 'title', 'body', 'parent_post', 'author', 'time_stamp', 'author_username')
        extra_kwargs = {'id': {'read_only': True},
                        'author': {'read_only': True}}

性能优化（可选） #

当查询大量Post数据时，为避免N+1查询问题，在视图中用select_related预取关联的author数据：

class PostList(generics.ListCreateAPIView):
    permission_classes = [IsAuthenticatedOrReadOnly]
    # 预取author关联对象，减少数据库查询次数
    queryset = models.Post.objects.all().select_related('author')
    serializer_class = serializers.PostSerializer

内容的提问来源于stack exchange，提问作者coderman