Get Inner JSON Value in Go

Simple question that I'm having a difficult time how to structure a struct for JSON decoding.

How can I copy an inner field of a struct to another field of a struct?

I have JSON

{
    "Trains": [{
        "Car": "6",
        "Destination": "SilvrSpg",
        "DestinationCode": "B08",
        "DestinationName": "Silver Spring",
        "Group": "1",
        "Line": "RD",
        "LocationCode": "A13",
        "LocationName": "Twinbrook",
        "Min": "1"
    }]
}

And I have structs

type Trains struct {
  Min      string `json:"Min"`
  DestName string `json:"DestinationName"`
  DestCode string `json:"DestinationCode"`
  LocName  string `json:"LocationName"`
  LocCode  string `json:"LocationCode"`
  Line     string `json:"Line"`
}

type AllData struct {
  Data []Trains `json:"Trains"`
}

How Can I get the value of the Trains.LocationCode to a struct like

type AllData struct {
  Id Trains[0].LocCode value
  Data []Trains `json:"Trains"`
}

So I basically just need to have JSON like this

{
    "Id":"A13",
    "Data": [{
        "Car": "6",
        "Destination": "SilvrSpg",
        "DestinationCode": "B08",
        "DestinationName": "Silver Spring",
        "Group": "1",
        "Line": "RD",
        "LocationCode": "A13",
        "LocationName": "Twinbrook",
        "Min": "1"
    }]
}

Where the Id is the inner value of the Trains struct.

How can I structure a struct to reflect this?

The JSON decoder does not have this capability. You must write the line of code in your application.

package main

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

var s = `
{
    "Trains": [{
        "Car": "6",
        "Destination": "SilvrSpg",
        "DestinationCode": "B08",
        "DestinationName": "Silver Spring",
        "Group": "1",
        "Line": "RD",
        "LocationCode": "A13",
        "LocationName": "Twinbrook",
        "Min": "1"
    }]
}`

type Train struct {
    Min      string `json:"Min"`
    DestName string `json:"DestinationName"`
    DestCode string `json:"DestinationCode"`
    LocName  string `json:"LocationName"`
    LocCode  string `json:"LocationCode"`
    Line     string `json:"Line"`
}

type Data struct {
    // The name "-" tells the JSON decoder to ignore this field.
    ID     string `json:"-"`
    Trains []Train
}

func main() {
    var d Data
    if err := json.Unmarshal([]byte(s), &d); err != nil {
        log.Fatal(err)
    }
    if len(d.Trains) < 1 {
        log.Fatal("No trains")
    }
    // Copy value from inner to outer.
    d.ID = d.Trains[0].LocCode
    fmt.Printf("%+v\n", &d)
}