Unmarshaling Nested JSON into Array of Objects in Go

Problem

Consider the following JSON data:

{
   "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
   }
}

The goal is to unmarshal this JSON into an array of Monster objects.

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

Solution

To achieve this, follow these steps:

1. Define MonsterCollection: Create a MonsterCollection type to hold a map of Monster objects with string keys.

type MonsterCollection struct {
    Pool map[string]Monster
}

2. Implement FromJson Method: Define a FromJson method on the MonsterCollection type to handle the unmarshaling.

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

• Using var data *map[string]Monster avoids the use of interface{}, ensuring that the resulting map has the desired type.
• Passing data as a pointer ensures that the changes made during unmarshaling are reflected in the MonsterCollection's Pool field.

3. Correct Key Type in Pool: Change the key type of the Pool map to string to match the keys in the JSON data.
4. Use Struct Tags: Add JSON struct tags to the Monster struct to specify the field names in the JSON data.

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

5. Call FromJson: Call the FromJson method with the JSON string to unmarshal the data into the MonsterCollection.

mc := new(MonsterCollection)
err := mc.FromJson(jsonStr)

6. Check for Errors: Handle any errors returned by FromJson for debugging purposes.
7. Access the Objects: Use the Pool field of the MonsterCollection to access the individual Monster objects.

for id, monster := range mc.Pool {
    fmt.Println(id, monster)
}

By following these steps, you can successfully unmarshal the nested JSON data into an array of Monster objects in Go.