根据Golang中的对象值对JSON的解析进行排序

Question

Trying to parse the json and sorting based on one of the values of struct. I want to sort the json based on the custom_meta's part_num, how we can do that. The code is as follows:

type Maininfo struct {
    Id   string     `json:"id"`
    Meta []Metainfo `json:"meta"`
}


type Metainfo struct {
    Filename     string `json:"filename"`
    Custom_meta  string `json:"custom_meta"`
    Size         int    `json:"size"`
    Content_hash string `json:"content_hash"`
}

type Custom_meta struct {
    Part_num string `json:"part_num"`
    Part     int
}

func getMeta(body []byte) (*Maininfo, error) {
    var s = new(Maininfo)
    err := json.Unmarshal(body, &s)
    if err != nil {
        fmt.Println("whoops:", err)
    }
    return s, err
}


func getMetainfo(body []byte) (*Metainfo, error) {
    var s = new(Metainfo)
    err := json.Unmarshal(body, &s)
    if err != nil {
        fmt.Println("error", err)
    }
    return s, err
}

type AxisSorter []Metainfo

func (a AxisSorter) Len() int           { return len(a) }
func (a AxisSorter) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a AxisSorter) Less(i, j int) bool { return a[i].Custom_meta < a[j].Custom_meta }


type NameSorter []Metainfo

func (a NameSorter) Len() int           { return len(a) }
func (a NameSorter) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a NameSorter) Less(i, j int) bool { return a[i].Custom_meta < a[j].Custom_meta }

func main() {
s, err := getMeta([]byte(body))
    fmt.Println("Main stuff", s)

    var metaInfo []Metainfo
    metaInfo = s.Meta
}
    var customMeta CustomMeta

    sort.Sort(AxisSorter(metaInfo))
    fmt.Println("metaInfo sorted ", metaInfo)

    sort.Sort(NameSorter(metaInfo))
    fmt.Println("metaInfo sorted 2 ", metaInfo)

    sort.Slice(metaInfo, func(i, j int) bool {
        fmt.Println("meta ", metaInfo[i].Custom_meta)
        return metaInfo[i].Custom_meta < metaInfo[j].Custom_meta
      })

}

I am not able to sort the code based on the part_num, how can we do that, since info is not a separate object it is a string. How we can parse string and sort it based on the int value.

Answer 1

It looks like the main issue here is that the "custom_meta" value is a quoted JSON string instead of a nested object, which means it cannot be unmarshaled into an object with the (presumably) desired "part_num" integer.

Ideally you could fix the source of this data so that it emits a JSON object instead of a quoted JSON string; however, if that is not feasible, then you could do the following.

1. Have the "Custom_meta" type implement json.Umarshaler by first unquoting the source string and then unmarshaling as usual.
2. Sort "Maininfo.Meta" by the nested "Custom_meta.Part" field separately or as part of a custom unmarshaler for that type.

For example (Go Playground):

type MainInfo struct {
  Id        string     `json:"id"`
  MetaInfos []MetaInfo `json:"meta"`
}

type MetaInfo struct {
  Filename    string     `json:"filename"`
  Custom      CustomMeta `json:"custom_meta"`
  Size        int        `json:"size"`
  ContentHash string     `json:"content_hash"`
}

type CustomMeta struct {
  PartNum int `json:"part_num"`
}

func (cm *CustomMeta) UnmarshalJSON(bs []byte) error {
  // Unquote the source string so we can unmarshal it.
  unquoted, err := strconv.Unquote(string(bs))
  if err != nil {
    return err
  }

  // Create an aliased type so we can use the default unmarshaler.
  type CustomMeta2 CustomMeta
  var cm2 CustomMeta2

  // Unmarshal the unquoted string and assign to the original object.
  if err := json.Unmarshal([]byte(unquoted), &cm2); err != nil {
    return err
  }
  *cm = CustomMeta(cm2)
  return nil
}

Then you can sort after parsing like so:

var doc MainInfo
err := json.Unmarshal([]byte(jsonstr), &doc)
if err != nil {
  panic(err)
}
sort.Slice(doc.MetaInfos, func(i, j int) bool {
  p1 := doc.MetaInfos[i].Custom.PartNum
  p2 := doc.MetaInfos[j].Custom.PartNum
  return p1 < p2
})

Of course, you could also perform the sorting as part of a custom UnmarshalJSON method for the "MainInfo" type.