django restframework serializer 增加自定义字段

class AutomationCaseDownloadSerializer(serializers.ModelSerializer):
    """
    下载用例读取数据序列
    """
    # api = AutomationCaseApiSerializer(many=True, read_only=True)
    updateTime = serializers.DateTimeField(format="%Y-%m-%d %H:%M:%S", required=False, read_only=True)
    # automationGroupLevelFirst = serializers.CharField(source='automationGroupLevelFirst.name')
    user = serializers.CharField(source="user.first_name")
    api = serializers.SerializerMethodField()

    class Meta:
        model = AutomationTestCase
        fields = ('caseName', 'user', 'updateTime', 'api')

    def get_api(self, obj):
        return AutomationCaseApiSerializer(
            AutomationCaseApi.objects.filter(automationTestCase=obj).order_by("id"),
            many=True
        ).data


class AutomationCaseDownSerializer(serializers.ModelSerializer):
    """
    下载用例读取数据序列
    """
    automationGroup = AutomationCaseDownloadSerializer(many=True, read_only=True)

    class Meta:
        model = AutomationGroupLevelFirst
        fields = ("name", "automationGroup")