In Go Language, how do I unmarshal json to array of object?

I have the following JSON, and I want to parse it into array of class:

{
	"1001": {"level":10, "monster-id": 1001, "skill-level": 1, "aimer-id": 301}
	"1002": {"level":12, "monster-id": 1002, "skill-level": 1, "aimer-id": 302}
	"1003": {"level":16, "monster-id": 1003, "skill-level": 2, "aimer-id": 303}
}

Here is what i am trying to do but failed:

type Monster struct {
	MonsterId  int32
	Level      int32
	SkillLevel int32
	AimerId    int32
}


type MonsterCollection struct {
	Pool map[string]Monster
}

func (mc *MonsterCollection) FromJson(jsonStr string) {
	var data interface{}
	b := []byte(jsonStr)
	err := json.Unmarshal(b, &data)
	if err != nil {
		return
	}

	m := data.(map[string]interface{})

	i := 0
	for k, v := range m {

		monster := new(Monster)
		monster.Level = v["level"]
		monster.MonsterId = v["monster-id"]
		monster.SkillLevel = v["skill-level"]
		monster.AimerId = v["aimer-id"]

		mc.Pool[i] = monster
		i++
	}

}

The compiler complain about the v["level"]
<< invalid operation. index of type interface().

This code has many errors in it. To start with, the json isn't valid json. You are missing the commas in between key pairs in your top level object. I added the commas and pretty printed it for you:

{
   "1001":{
      "level":10,
      "monster-id":1001,
      "skill-level":1,
      "aimer-id":301
   },
   "1002":{
      "level":12,
      "monster-id":1002,
      "skill-level":1,
      "aimer-id":302
   },
   "1003":{
      "level":16,
      "monster-id":1003,
      "skill-level":2,
      "aimer-id":303
   }
}

Your next problem (the one you asked about) is that m := data.(map[string]interface{}) makes m a map[string]interface{}. That means when you index it such as the v in your range loop, the type is interface{}. You need to type assert it again with v.(map[string]interface{}) and then type assert each time you read from the map.

I also notice that you next attempt mc.Pool[i] = monster when i is an int and mc.Pool is a map[string]Monster. An int is not a valid key for that map.

Your data looks very rigid so I would make unmarshall do most of the work for you. Instead of providing it a map[string]interface{}, you can provide it a map[string]Monster.

Here is a quick example. As well as changing how the unmarshalling works, I also added an error return. The error return is useful for finding bugs. That error return is what told me you had invalid json.

type Monster struct {
	MonsterId  int32 `json:"monster-id"`
	Level      int32 `json:"level"`
	SkillLevel int32 `json:"skill-level"`
	AimerId    int32 `json:"aimer-id"`
}

type MonsterCollection struct {
	Pool map[string]Monster
}

func (mc *MonsterCollection) FromJson(jsonStr string) error {
	var data = &mc.Pool
	b := []byte(jsonStr)
	return json.Unmarshal(b, data)
}

I posted a working example to goplay: http://play.golang.org/p/4EaasS2VLL

Slightly off to one side - you asked for an array of objects when you needed a map

If you need an array (actually a slice)

http://ioblocks.blogspot.com/2014/09/loading-arrayslice-of-objects-from-json.html