How to Efficiently Unmarshal JSON into an Array of Go Structs?

Unmarshalling JSON into an Array of Objects in Go

You encounter a challenge while attempting to parse a JSON object into an array of custom structs. The JSON you provided, however, appears to be invalid as it lacks commas between key-value pairs in the top-level object. Here's the corrected and formatted JSON:

{
   "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
   }
}

Parsing with Type Assertions

Your initial attempt to parse the JSON encountered an error because you assigned the map resulting from JSON unmarshalling to an interface{} value. When indexing this map, you need to type assert the value again with v.(map[string]interface{}) and each time you retrieve a key from the map.

Additionally, you attempted to insert an int as the key of a map with strings as keys, which would result in a further error.

Efficient Unmarshalling

To simplify the unmarshalling process and avoid manual type assertions, you can use a technique where you provide unmarshalling with the target data type directly. Here's an updated example:

package main

import (
    "encoding/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)
}

func main() {
    jsonString := `{
        "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
        }
    }`

    mc := MonsterCollection{}
    if err := mc.FromJson(jsonString); err != nil {
        // Handle error
    }

    for _, monster := range mc.Pool {
        // Access monster data here
    }
}

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