How to Unmarshal JSON Data with Tagged Fields in Go?

How to Unmarshal JSON with Tagged Fields

When unmarshaling JSON into a struct, it may be necessary to specify how certain fields are handled. To do this, tags can be added to the struct fields to provide additional information to the unmarshaling process.

In a scenario where you need to unmarshal a JSON field as a string into a struct field with a tag, a simple solution using reflection can be implemented:

<code class="go">package main

import (
    "encoding/json"
    "fmt"
    "log"
    "reflect"
)

type A struct {
    I int64
    S string `sql:"type:json"`
}

const data = `{
    "I": 3,
    "S": {
        "phone": {
            "sales": "2223334444"
        }
    }
}`

func main() {
    var a A
    err := json.Unmarshal([]byte(data), &a)
    if err != nil {
        log.Fatal("Unmarshal failed", err)
    }

    rt := reflect.TypeOf(a)
    rv := reflect.ValueOf(&a)
    for i := 0; i < rt.NumField(); i++ {
        f := rt.Field(i)
        if f.Tag.Get("json") != "" {
            fv := rv.Elem().Field(i)
            fv.SetString(string(fv.Bytes()))
        }
    }

    fmt.Println("Done", a)
}</code>

However, a more elegant approach is available in Go that eliminates the need for reflection:

<code class="go">package main

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

// RawString is a raw encoded JSON object.
// It implements Marshaler and Unmarshaler and can
// be used to delay JSON decoding or precompute a JSON encoding.
type RawString string

// MarshalJSON returns *m as the JSON encoding of m.
func (m *RawString) MarshalJSON() ([]byte, error) {
    return []byte(*m), nil
}

// UnmarshalJSON sets *m to a copy of data.
func (m *RawString) UnmarshalJSON(data []byte) error {
    if m == nil {
        return errors.New("RawString: UnmarshalJSON on nil pointer")
    }
    *m += RawString(data)
    return nil
}

const data = `{"i":3, "S":{"phone": {"sales": "2223334444"}}}`

type A struct {
    I int64
    S RawString `sql:"type:json"`
}

func main() {
    a := A{}
    err := json.Unmarshal([]byte(data), &a)
    if err != nil {
        log.Fatal("Unmarshal failed", err)
    }
    fmt.Println("Done", a)
}</code>

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