Question 170883 in Json Question posted in
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Why does JSON number get cut off after unmarshaling to interface?

So I have a JSON with many fields and I am looping through it as suggested by
How effectively to change JSON keys
to delete some of the keys I don’t need. But after deletion, the original values of the existing JSON was changed, some of them are float numbers it seems and I made a demo to show it.

How can I change this behavior? Is the interface{} causing the issue? Why is 1684366653200744506 cut off to 1684366653200744400?

Thanks!

https://go.dev/play/p/X2auWqWB2fL

For reference, the output JSON is changed to 1684366653200744400

2009/11/10 23:00:00 1684366653200744448.000000
2009/11/10 23:00:00 map[timestamp:1.6843666532007444e+18]
2009/11/10 23:00:00 json Marshal from maps of key string and value interface to batch json for insert to DB
2009/11/10 23:00:00 {"timestamp":1684366653200744400}
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2

Answers


  1. 0

    I suggest creating a type and removing the field you don’t need.

    package main

import (
    "encoding/json"
    "log"
)

type A struct {
    Timestamp int64 `json:"timestamp"`
}

func main() {
    jsonBatch := `{"timestamp":1684366653200744506, "todelete":"string value or boolean value"}`
    i := A{}
    if err := json.Unmarshal([]byte(jsonBatch), &i); err != nil {
        log.Println("json UnMarshal from batch json failed")
        log.Println(err)
    }
    dbJsonBatch, err := json.Marshal(i)
    if err != nil {
        log.Println("json Marshal from batch json failed")
        log.Println(err)
    }
    log.Println("json Marshal from maps of key string and value interface to batch json for insert to DB")
    log.Println(string(dbJsonBatch))
}

    This prints

    2009/11/10 23:00:00 json Marshal from maps of key string and value interface to batch json for insert to DB
2009/11/10 23:00:00 {"timestamp":1684366653200744506}
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  2. 0

    It’s because by default, the encoding/json package stores float64 in the interface value for JSON numbers. See json.Unmarshal:

    To unmarshal JSON into an interface value, Unmarshal stores one of these in the interface value:

    • bool, for JSON booleans
    • float64, for JSON numbers
    • …

    You can create a decoder and call (*Decoder).UseNumber to change the behavior:

    jsonBatch := `{"timestamp":1684366653200744506, "todelete":"string value or boolean value"}`
dec := json.NewDecoder(strings.NewReader(jsonBatch))
dec.UseNumber()
var i interface{}
if err := dec.Decode(&i); err != nil {

    See https://go.dev/play/p/ZjWB-NfiEQL.

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