DataWeave技术咨询：迭代列表后添加UsagePoint元素报错如何解决

我想要生成如下结构的XML：

<ns10:Payload>
   <ns6:MeterReadings>
      <ns6:MeterReading>
         <ns6:Readings>
            <ns6:reportedDateTime>2024-06-02T23:15:11.09+02:00</ns6:reportedDateTime>
            <ns6:timeStamp>2024-06-02T23:14:39+02:00</ns6:timeStamp>
            <ns6:value>0.234</ns6:value>
            <ns6:ReadingType ref="0.0.0.6.0.1.54.0.0.0.0.0.0.0.128.3.29.0"/>
         </ns6:Readings>
         <ns6:Readings>
            <ns6:reportedDateTime>2024-06-02T23:15:11.09+02:00</ns6:reportedDateTime>
            <ns6:timeStamp>2024-06-02T23:14:39+02:00</ns6:timeStamp>
            <ns6:value>0.233</ns6:value>
            <ns6:ReadingType ref="0.0.0.6.0.1.54.0.0.0.0.0.0.0.64.3.29.0"/>
         </ns6:Readings>
         <ns6:UsagePoint>
            <ns6:mRID>ES00TRAFGISS03268T120F</ns6:mRID>
         </ns6:UsagePoint>
      </ns6:MeterReading>
   </ns6:MeterReadings>
</ns10:Payload>

我尝试使用++操作符在列表末尾添加UsagePoint元素，但Mule报错提示数组与对象类型不兼容。我的DataWeave代码如下：

%dw 2.0
output application/xml
ns ns0 http://iec.ch/TC57/2011/MeterReadingsMessage
ns ns01 http://iec.ch/TC57/2011/schema/message
ns ns02 http://iec.ch/TC57/2011/MeterReadings

var meterReadings = payload.MeterReadings
var meterReading = meterReadings.MeterReading
var readings = meterReading.readings[0] as Array<T>

---
ns0#CreatedMeterReadings: {
    ns0#Header: {
        ns01#Verb: "reply",
        ns01#Noun: "MeterReadings"
    },
    ns0#Payload: {
        ns02#MeterReadings: {
            ns02#MeterReading: readings map ((reading) -> {
                ns02#Readings: {
                    ns02#reportedDateTime: reading.reportedDateTime,
                    ns02#timeStamp: reading.timeStamp,
                    ns02#value: reading.value,
                    ns02#ReadingType: reading.ReadingType
                } 
            })
            reduce ($$ ++ $) 
            ++ ns02#UsagePoint: {
                ns02#mRID: "example"
            }
        }
    }
}

请问是否可以实现我想要的效果？如果可以，该如何修改代码？

可以实现，问题根源是你试图直接将对象类型的UsagePoint与reduce后的数组拼接，导致类型不兼容。正确的做法是把UsagePoint包装成数组元素，再和Readings数组合并，XML会自动将数组中的每个对象渲染为同级子元素。

修改后的DataWeave代码：

%dw 2.0
output application/xml
ns ns0 http://iec.ch/TC57/2011/MeterReadingsMessage
ns ns01 http://iec.ch/TC57/2011/schema/message
ns ns02 http://iec.ch/TC57/2011/MeterReadings

var meterReadings = payload.MeterReadings
var meterReading = meterReadings.MeterReading
var readings = meterReading.readings[0] as Array<T>

---
ns0#CreatedMeterReadings: {
    ns0#Header: {
        ns01#Verb: "reply",
        ns01#Noun: "MeterReadings"
    },
    ns0#Payload: {
        ns02#MeterReadings: {
            ns02#MeterReading: (readings map ((reading) -> {
                ns02#Readings: {
                    ns02#reportedDateTime: reading.reportedDateTime,
                    ns02#timeStamp: reading.timeStamp,
                    ns02#value: reading.value,
                    ns02#ReadingType: reading.ReadingType
                } 
            }) 
            // 将UsagePoint包装为单元素数组，再和Readings数组合并
            ++ [{
                ns02#UsagePoint: {
                    ns02#mRID: "ES00TRAFGISS03268T120F"
                }
            }])
        }
    }
}

关键修改说明： #

• 移除冗余的reduce操作：map本身会返回数组，无需额外用reduce拼接。
• 把UsagePoint对象放入单元素数组中，确保++两侧都是数组类型，解决类型不兼容问题。
• XML输出时，数组内的每个对象会自动成为ns02#MeterReading的同级子元素，完全匹配目标结构。

内容的提问来源于stack exchange，提问作者mripoll