用分片对结构进行解组将返回空值而不是空分片 [英] Unmarshal to struct with slice returns null value instead of empty slice

问题描述

如果我创建没有任何标签的照片"，则将其存储为dynamodb中的

If I create a "photo" without any tags it is stored in dynamodb as

"tags": {
   "NULL": true
}, 

但是当我查询并解组记录时，我希望它会将其转换成一个空片，而不是我得到这个:

But when I query and unmarshal the record I would expect that it converts this into an empty slice instead I get this:

[{"photo_id":"bmpuh3jg","tags":null}]

是否可以将其转换为空切片?例如

Is it possible to have it convert it into an empty slice instead? e.g.

[{"photo_id":"bmpuh3jg","tags":[]}]

代码示例

我的结构

CODE EXAMPLE

My struct

type Photo struct {
    Id        string   `json:"photo_id"`
    Tags      []string `json:"tags"`
}

查询

photo := &Photo{}
input := &dynamodb.QueryInput{
    TableName:                 aws.String("local.photos"),
    KeyConditionExpression:    aws.String("photo_id = :photo_id"),
    ExpressionAttributeValues: map[string]*dynamodb.AttributeValue{
        ":photo_id": {
            S: aws.String(photo_id),
        },
    },
}
db_result, err := db.Query(input)
if err != nil {
    return nil, err
} else if *db_result.Count == int64(0) {
    // No item found
    return nil, err
}

err = dynamodbattribute.UnmarshalListOfMaps(db_result.Items, photo)
if err != nil {
    return nil, err
}

photoJSON, err := json.Marshal(photo)
if err != nil {
    return nil, err
}

return photoJSON, nil

推荐答案

如果我正确理解了您的问题，要获得一个空白的标签({"photo_id":"bmpuh3jg","tags":[]})结果，您可以这样做:

If I understand your question correctly, to achieve a result with an empty slice for Tags ({"photo_id":"bmpuh3jg","tags":[]}), you can do it like this:

  jsonString := `{"photo_id":"bmpuh3jg","tags":null}`

  photo := &Photo{}
  err := json.Unmarshal([]byte(jsonString), &photo)
  if err != nil {
     fmt.Println(err.Error())
  }

  // Here is a trick. Replace nil with an empty slice.
  if photo.Tags == nil {
    photo.Tags = []string{}
  }
  elemJSON, err := json.Marshal(photo)
  if err != nil {
    fmt.Println(err.Error())
  }
  fmt.Println(string(elemJSON)) //{"photo_id":"bmpuh3jg","tags":[]} 

要了解为什么将零片段编码为空JSON，您可以查看官方文档 https://golang.org/pkg/encoding/json/

To understand, why a nil slice encodes as the null JSON, you can check official documentation https://golang.org/pkg/encoding/json/

数组和切片值编码为JSON数组，除了[] byte 编码为base64编码的字符串，而nil slice编码为 空JSON值.

Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON value.

在Go Playground上检查: https://play.golang.org/p/BsxTpBlypV5

Check on Go Playground: https://play.golang.org/p/BsxTpBlypV5

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