How to Avoid Double Unmarshaling When Parsing JSON in Go?

Unmarshalling JSON in Go Without Double Unmarshaling

In Go, it's possible to face a situation where you need to parse JSON into various structs, but double unmarshalling occurs, leading to redundancy. This article presents a solution that involves partially unmarshalling the data to avoid the second unmarshaling step.

Understanding the Issue

When attempting to parse JSON into known structs, a common approach is:

1. Unmarshal JSON into a generic map[string]interface{}
2. Find the expected key
3. Attempt to cast the value to the desired type
4. If casting fails, marshal the value as JSON and unmarshal it to the desired type (double unmarshalling)

The Solution

Instead of casting the value to an interface{}, one can unmarshal it into a json.RawMessage. This partially unmarshals the data and allows subsequent unmarshalling to specific structs based on the key value.

Sample Code

import (
    "encoding/json"
    "fmt"
)

type Ping struct {
    Ping string `json:"ping"`
}

type Ack struct {
    Messages []Message `json:"messages"`
}

type Message string

func main() {
    testJSON := []byte(`{"ack":{"messages":["Hi there","Hi again"]}}`)
    var myAck = Ack{}
    var myMap map[string]json.RawMessage

    err := json.Unmarshal(testJSON, &myMap)
    if err != nil {
        fmt.Println("error unmarshalling: ", err)
    }

    jsonAck, ok := myMap["ack"]
    if !ok {
        fmt.Println("key 'ack' not found")
        return
    }

    err = json.Unmarshal(jsonAck, &myAck)
    if err != nil {
        fmt.Println("unmarshal error", err)
    } else {
        fmt.Println("New ack object: ", myAck)
    }
}

In this modified code, we replace the map[string]interface{} with map[string]json.RawMessage. When handling the "ack" key, we unmarshal directly to the Ack struct, eliminating the need for double unmarshalling.

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