How to decode json into structs

I'm trying to decode some json in Go but some fields don't get decoded.
See the code running in browser here:

What am I doing wrong?

I need only the MX records so I didn't define the other fields. As I understand from the godoc you don't need to define the fields you don't use/need.

// You can edit this code!
// Click here and start typing.
package main
import "fmt"
import "encoding/json"
func main() {
body := `
{"response": {
"status": "SUCCESS",
"data": {
"mxRecords": [
{
"value": "us2.mx3.mailhostbox.com.",
"ttl": 1,
"priority": 100,
"hostName": "@"
},
{
"value": "us2.mx1.mailhostbox.com.",
"ttl": 1,
"priority": 100,
"hostName": "@"
},
{
"value": "us2.mx2.mailhostbox.com.",
"ttl": 1,
"priority": 100,
"hostName": "@"
}
],
"cnameRecords": [
{
"aliasHost": "pop.a.co.uk.",
"canonicalHost": "us2.pop.mailhostbox.com."
},
{
"aliasHost": "webmail.a.co.uk.",
"canonicalHost": "us2.webmail.mailhostbox.com."
},
{
"aliasHost": "smtp.a.co.uk.",
"canonicalHost": "us2.smtp.mailhostbox.com."
},
{
"aliasHost": "imap.a.co.uk.",
"canonicalHost": "us2.imap.mailhostbox.com."
}
],
"dkimTxtRecord": {
"domainname": "20a19._domainkey.a.co.uk",
"value": "\"v=DKIM1; g=*; k=rsa; p=DkfbhO8Oyy0E1WyUWwIDAQAB\"",
"ttl": 1
},
"spfTxtRecord": {
"domainname": "a.co.uk",
"value": "\"v=spf1 redirect=_spf.mailhostbox.com\"",
"ttl": 1
},
"loginUrl": "us2.cp.mailhostbox.com"
}
}}`
type MxRecords struct {
value    string
ttl      int
priority int
hostName string
}
type Data struct {
mxRecords []MxRecords
}
type Response struct {
Status string `json:"status"`
Data   Data   `json:"data"`
}
type apiR struct {
Response Response
}
var r apiR
err := json.Unmarshal([]byte(body), &r)
if err != nil {
fmt.Printf("err was %v", err)
}
fmt.Printf("decoded is %v", r)
}

As per the go documentaiton about json.Unmarshal, you can only decode toward exported fields, the main reason being that external packages (such as encoding/json) cannot acces unexported fields.

If your json doesn't follow the go convention for names, you can use the json tag in your fields to change the matching between json key and struct field.

Exemple:

package main
import (
"fmt"
"encoding/json"
)
type T struct {
Foo  string `json:"foo"`
priv string `json:"priv"`
}
func main() {
text := []byte(`{"foo":"bar", "priv":"nothing"}`)
var t T
err := json.Unmarshal(text, &t)
if err != nil {
panic(err)
}
fmt.Println(t.Foo) // prints "bar"
fmt.Println(t.priv) // prints "", priv is not exported
}

You must Uppercase struct fields:

type MxRecords struct {
Value    string `json:"value"`
Ttl      int    `json:"ttl"`
Priority int    `json:"priority"`
HostName string `json:"hostName"`
}
type Data struct {
MxRecords []MxRecords `json:"mxRecords"`
}

http://play.golang.org/p/EEyiISdoaE

The encoding/json package can only decode into exported struct fields. Your Data.mxRecords member is not exported, so it is ignored when decoding. If you rename it to use a capital letter, the JSON package will notice it.

You will need to do the same thing for all the members of your MxRecords type.