Golang remove string prefix when unmarshalling JSON

Question

In my json data structure I have a string that can have a prefix. When unmarshalling JSON, is it possible to have a function to remove that prefix? I am looking into custom JSON unmarshalling in golang and trying to leverage that.

For example. The payload can be either of the following

{
  "id": "urn:uuid:1234567890"
}

{
  "id": "1234567890"
}

When I do JSON.unmarshall(data, &struct) I'd like the unmarshall function to handle removing the urn:uuid prefix from the string if it is there so the struct will always have the value 1234567890 for id .

Answer 1

You can provide a custom UnmarshalJSON method on the data you need to trim, here is an example implementation, you may need to extend if you have to regex match the start rather than match hard string (or byte array in this case):

go playground

package main

import (
    "bytes"
    "encoding/json"
    "log"
)

var (
    sampleJSON   = []byte(`{"id": "urn:uuid:1234567890"}`)
    prefixToTrim = []byte(`urn:uuid:`)
)

type IDField string

type Data struct {
    ID IDField `json:"id"`
}

func main() {
    d := &Data{}
    err := json.Unmarshal(sampleJSON, d)
    if err != nil {
        log.Fatal(err)
    }

    log.Println(d.ID)
}

// UnmarshalJSON provides custom unmarshalling to trim `urn:uuid:` prefix from IDField
func (id *IDField) UnmarshalJSON(rawIDBytes []byte) error {

    // trim quotes and prefix
    trimmedID := bytes.TrimPrefix(bytes.Trim(rawIDBytes, `"`), prefixToTrim)

    // convert back to id field & assign
    *id = IDField(trimmedID)
    return nil
}