Unmarshal JSON into a map in Go

Question

I'm having trouble figuring out how to load a "subsection" of JSON file into a map element. Background: I'm trying to unmarshal a somewhat complicated configuration file which has a strict structure, so I assume it's preferable to unmarshal into a "static" structure rather than into an interface{}.

Here's a simple JSON file for example:

{
    "set1": {
        "a":"11",
        "b":"22",
        "c":"33"
    }
}

This code works:

package main

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
    "os"
)

type JSONType struct {
    FirstSet ValsType `json:"set1"`
}

type ValsType struct {
    A string `json:"a"`
    B string `json:"b"`
    C string `json:"c"`
}

func main() {
    file, e := ioutil.ReadFile("./test1.json")
    if e != nil {
        fmt.Println("file error")
        os.Exit(1)
    }
    var s JSONType
    json.Unmarshal([]byte(file), &s)
    fmt.Printf("\nJSON: %+v\n", s)
}

But this doesn't:

package main

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
    "os"
)

type JSONType struct {
    FirstSet ValsType `json:"set1"`
}

type ValsType struct {
    Vals map[string]string
}

func main() {
    file, e := ioutil.ReadFile("./test1.json")
    if e != nil {
        fmt.Println("file error")
        os.Exit(1)
    }

    var s JSONType
    s.FirstSet.Vals = map[string]string{}
    json.Unmarshal([]byte(file), &s)
    fmt.Printf("\nJSON: %+v\n", s)
}

The Vals map isn't loaded. What am I doing wrong? Thanks for any help!

Here's a better example:

{
    "set1": {
        "a": {
            "x": "11",
            "y": "22",
            "z": "33"
        },
        "b": {
            "x": "211",
            "y": "222",
            "z": "233"
        },
        "c": {
            "x": "311",
            "y": "322",
            "z": "333"
        },
    }
}

Code:

package main

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
    "os"
)

type JSONType struct {
    FirstSet map[string]ValsType `json:"set1"`
}

type ValsType struct {
    X string `json:"x"`
    Y string `json:"y"`
    Z string `json:"z"`
}

func main() {
    file, e := ioutil.ReadFile("./test1.json")
    if e != nil {
        fmt.Println("file error")
        os.Exit(1)
    }
    var s JSONType
    json.Unmarshal([]byte(file), &s)
    fmt.Printf("\nJSON: %+v\n", s)
}

Answer 1

I believe that is because you have extra layer of indirection in your models.

type JSONType struct {
    FirstSet map[string]string `json:"set1"`
}

Should suffice. if you specify map[string]string the object in json is recognized as that map. You created a struct to wrap it but a blob of json like this;

{
    "a":"11",
    "b":"22",
    "c":"33"
}

Actually can unmarshal directly into map[string]string

EDIT: Some other models based on the comment

type JSONType struct {
    FirstSet map[string]Point `json:"set1"`
}

type Point struct {
     X string `json:"x"`
     Y string `json:"y"`
     Z string `json:"z"`
}

This makes your 3-d point a statically typed struct which is fine. If you wanted to do the quick and dirty you could also just use map[string]map[string]string which would give a map of maps so you could access the point values like FirstSet["a"]["x"] and it would return "11".

Second edit; clearly I didn't read you code that closely since the above example is the same. Based on that I would guess you want the

 FirstSet map[string]map[string]string `json:"set1"`

option. Though it's not entirely clear to me after your edit.