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

Question

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().

Answer 1

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