如何为Django Serializer类设置懒引用解决循环依赖类未定义错误

Python类定义是从上到下顺序执行的，你在定义QuestionAnsweringModelResultsSerializer时，它引用的FillInTheBlanksAnsweringModelResultsSerializer还没有完成定义，直接在类属性中引用未定义的类就会抛出未定义错误，本质和跨文件循环导入一致。

推荐使用原生支持的动态字段注入方式，无需额外依赖，稳定性最高：
将互相引用的字段从类属性中移除，改为在序列化器的__init__方法中动态添加到字段列表中，此时两个序列化器类都已经完成定义，不会出现未定义问题，修改后的代码如下：

class QuestionAnsweringModelResultsSerializer(ModelSerializer):
    answers = AnswerAnsweringModelResultsSerializer(many=True)
    images = QuestionAnsweringModelImagesResultsSerializer(many=True)
    written_answer_marking_settings = QuestionMarkingSettingsSerializer()
    parent_question = QuestionResultsSerializer()
    procedural_answers = ProceduralAnswerAnsweringSerializer(many=True)

    class Meta:
        model = QuestionAnsweringModel
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # 运行到此处时，下方的FillInTheBlanksAnsweringModelResultsSerializer已经定义完成
        self.fields['fill_in_the_blanks_answers'] = FillInTheBlanksAnsweringModelResultsSerializer(many=True)


class FillInTheBlanksAnsweringModelResultsSerializer(ModelSerializer):
    class Meta:
        model = FillInTheBlanksAnsweringModel
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # 运行到此处时，上方的QuestionAnsweringModelResultsSerializer已经定义完成
        self.fields['question'] = QuestionAnsweringModelResultsSerializer()

如果你不想修改__init__方法，也可以使用SerializerMethodField实现按需序列化，示例如下：

from rest_framework.fields import SerializerMethodField

class QuestionAnsweringModelResultsSerializer(ModelSerializer):
    answers = AnswerAnsweringModelResultsSerializer(many=True)
    images = QuestionAnsweringModelImagesResultsSerializer(many=True)
    written_answer_marking_settings = QuestionMarkingSettingsSerializer()
    parent_question = QuestionResultsSerializer()
    procedural_answers = ProceduralAnswerAnsweringSerializer(many=True)
    fill_in_the_blanks_answers = SerializerMethodField()

    def get_fill_in_the_blanks_answers(self, obj):
        return FillInTheBlanksAnsweringModelResultsSerializer(obj.fill_in_the_blanks_answers.all(), many=True, context=self.context).data

    class Meta:
        model = QuestionAnsweringModel
        fields = '__all__'


class FillInTheBlanksAnsweringModelResultsSerializer(ModelSerializer):
    question = SerializerMethodField()

    def get_question(self, obj):
        return QuestionAnsweringModelResultsSerializer(obj.question, context=self.context).data

    class Meta:
        model = FillInTheBlanksAnsweringModel
        fields = '__all__'

不过这种方法返回的是序列化后的字典，不是序列化器字段实例，部分DRF特性比如自动生成OpenAPI文档会受影响，因此更推荐第一种方法。

内容的提问来源于stack exchange，提问作者Freduah Gideon