django REST框架多源领域

假设我的 models.py 中有这些：

#models.py
class Theme(models.Model):
    """An theme is an asset of multiple levels."""

    adventure = models.ForeignKey(Adventure)
    offset = models.PositiveSmallIntegerField()
    finished = models.BooleanField(default=False)

class Level(models.Model):
    """Abstract level representation"""

    theme = models.ForeignKey(Theme)
    offset = models.PositiveSmallIntegerField()
    finished = models.BooleanField(default=False)

    class Meta:
        abstract = True

class PuzzleLevel(Level):
    """A level for the Puzzle game"""

    points = models.CharField(max_length=200)
    image = models.ImageField()

class ImageAndWordLevel(Level):
    """A level for the ImageAndWord game"""

    word = models.CharField(max_length=30)
    image = models.ImageField()

我想在我的 api 中使用 Theme。
为此，我需要将其序列化：

#serializers.py
class PuzzleLevelSerializer(serializers.ModelSerializer):
    image = serializers.Field(source="image.url")

    class Meta:
        model = PuzzleLevel
        fields = ("offset", "finished", "points", "image")

class ImageAndWordLevelSerializer(serializers.ModelSerializer):
    image = serializers.Field(source="image.url")

    class Meta:
        model = ImageAndWordLevel
        fields = ("offset", "finished", "word", "image")

class ThemeSerializer(serializers.ModelSerializer):
    levels = serializers.Field(source="level_set")

    class Meta:
        model = Theme
        fields = ("offset", "finished", "levels")

不幸的是，我不能使用水平集源，因为它不存在。
我怎样才能结合谜题关卡集 and 图像和单词级别集在一个单一的levels field ?

实际上，在深入研究 REST 框架的源代码之后，我找到了一个解决方案：
我编写了一个能够管理多个源的自定义字段。

class MultiSourceField(serializers.Field):
    """
    A custom field to use when you want to
    join multiple sources into a single field.
    Example :
        my_field = MultiSourceField(source=["attr1.subattr", "attr2.subattr"])
    It can also handle serializers, a small example :
        rel_field = MultiSourceField(source=[Serializer(source="rel"), "attr2"])
    """

    def field_to_native(self, obj, field_name):
        if obj is None:
            return self.empty

        sources = self.source
        value = []
        for source in sources:
            if isinstance(source, serializers.BaseSerializer):
                value += source.field_to_native(obj, "")
            else:
                #setting self.source to source in order to use the parent method
                self.source = source
                value.append(super(MultiSourceField, self).field_to_native(obj,
                    field_name))
                #reverting self.source after the parent method call
                self.source = sources
        return value

现在，我的主题序列化器如下所示：

class ThemeSerializer(serializers.ModelSerializer):
    levels = MultiSourceField(source=[
        PuzzleLevelSerializer(source="puzzlelevel_set"),
        ImageAndWordLevelSerializer(source="imageandwordlevel_set")
    ])

    class Meta:
        model = Theme
        fields = ("offset", "finished", "levels",)