How Can I Efficiently Handle Type Assertion After Unmarshalling JSON into interface{} in Go?

Unmarshalling into Interface{} for Type Assertion in Golang

In this scenario, JSON strings are transmitted via RabbitMQ. These strings represent instances of two different structs, Somthing1 and Somthing2. The goal is to unmarshal the JSON strings back into their respective structs and perform type assertions.

The Problem

At first glance, it seems that unmarshalling into an interface{} and then applying type assertions should suffice. However, upon unmarshalling, the input variable is of type map[string]interface{}. This is not as expected, and attempts to switch on this type or reassign it to the desired structs fail.

The Solution

The JSON library in Golang unmarshals into default types such as bool, float64, and map[string]interface{}. To obtain the desired structs, you need to unmarshal directly into them or manually convert from the map[string]interface{}.

Using Direct Unmarshal

The preferred approach is to unmarshal directly into the structs:

func typeAssert(msg string) {
    var job Somthing1
    json.Unmarshal([]byte(msg), &job)

    // ...

    var stats Somthing2
    json.Unmarshal([]byte(msg), &stats)

    // ...
}

Using an Unpacker Struct

If unmarshalling directly is not feasible, you can use an Unpacker struct to handle the unmarshalling and provide an interface for accessing the data:

type Unpacker struct {
    Data       interface{}
}

func (u *Unpacker) UnmarshalJSON(b []byte) error {
    // Attempt to unmarshal into both types
    smth1 := &Something1{}
    err := json.Unmarshal(b, smth1)

    if err == nil && smth1.Thing != "" {
        u.Data = smth1
        return nil
    }

    smth2 := &Something2{}
    err = json.Unmarshal(b, smth2)
    if err != nil {
        return err
    }

    u.Data = smth2
    return nil
}

Then, you can use the Unpacker to access the unmarshalled data:

func typeAssert(msg string) {
    unpacker := &Unpacker{}
    json.Unmarshal([]byte(msg), unpacker)

    switch v := unpacker.Data.(type) {
    case Something1:
        // ...

    case Something2:
        // ...

    default:
        // Handle unknown type
    }
}

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