Json data adapter - go

Question

I want to take JSON in
{ name: "http://example.com", ...}
and map it to
{ url: "http://example.com", ...}
I want to do this mapping and be able to Marshal back to json as Url. The struct is rather large, so is it possible to do this with one struct instead creating multiple struct types?
This is the solution I have come to so far: http://play.golang.org/p/wBPbSjkTYF
After seeing answers and how I failed to accurately describe my use case. Updating to the actual mapping I'm trying to do:
{ "data": { "name": "http://example.com", "key": "value" } }
To:
{ "url": "http://example.com", { "data": "key": "value" } }

Answer 1

The json package doesn't have any tags that allows you to use one key name of a field on Marshal and a different on Unmarshal.
It is still possible to Marshal a struct to differently, but in order to do that you will have to implement your own Marshaller (MarshalJSON function). A working example:
package main
import (
"bytes"
"encoding/json"
"fmt"
)
type Data struct {
Url string `json:"name"`
}
// marshalObject takes a slice of key values
// (should be correctly escaped json strings without "")
// and another slice of interface{} values and marshals
// them into a JSON object string
func marshalObject(keys []string, values []interface{}) ([]byte, error) {
if len(keys) != len(values) {
panic("Different length of keys and values slices")
}
if len(keys) == 0 {
return []byte(`{}`), nil
}
var b bytes.Buffer
b.Write([]byte(`{"`))
for i, key := range keys {
if i != 0 {
b.Write([]byte(`,"`))
}
b.WriteString(key)
b.Write([]byte(`":`))
j, err := json.Marshal(values[i])
if err != nil {
return nil, err
}
b.Write(j)
}
b.Write([]byte(`}`))
return b.Bytes(), nil
}
func (d *Data) MarshalJSON() ([]byte, error) {
// Here you can add a list of keys and values.
// Currently it is only `url` mapped to d.Url
return marshalObject(
[]string{
`url`,
},
[]interface{}{
d.Url,
},
)
}
func main() {
i := []byte(`{"name":"http://example.com"}`)
fmt.Printf("Json Input: %+s\n", i)
var d *Data
err := json.Unmarshal(i, &d)
if err != nil {
panic(err)
}
fmt.Printf("Data: %#v\n", d)
o, err := json.Marshal(d)
if err != nil {
panic(err)
}
fmt.Printf("Json Output: %+s\n", o)
}
Result:
Json Input: {"name":"http://example.com"}
Data: &main.Data{Url:"http://example.com"}
Json Output: {"url":"http://example.com"}
Playground:
http://play.golang.org/p/u6ExI9V95D

Answer 2

If you unmarshal a JSON object into an map[string]interface{} variable, you will be given everything in the object rather than just a few fields. It should be fairly straight forward to make the required modifications, like so:
func nameToUrl(data []byte) ([]byte, error) {
var obj map[string]interface{}
if err := json.Unmarshal(data, &obj); err != nil {
return nil, err
}
if name, ok := obj["name"]; ok {
delete(obj, "name")
obj["url"] = name
}
return json.Marshal(obj)
}
You can experiment with this here: http://play.golang.org/p/0jz_HAGg3E