How Can I Marshal JSON into a Go Interface{} and Dynamically Convert it to a Specific Type?

Marshaling JSON into an Interface in Go

When dealing with complex data structures, it's often necessary to marshal JSON into an interface{} to enable flexibility and polymorphism. This article provides a solution to a specific scenario where you want to marshal JSON into an interface{} and dynamically convert it to a specific type based on a field's value.

Problem Description

Consider the following situation: you have a Message type with an interface{} field called Data. You also have a CreateMessage type that represents a specific variant of the Data field. When unmarshaling a JSON string containing both a command ("create" in this case) and data that matches the CreateMessage type, you encounter the issue of Data remaining as an interface{}.

Solution

To resolve this issue, you need to leverage Go's type system and JSON's flexibility. Define a new struct type, Message, which embeds the original Message type and adds a RawMessage field called Data. RawMessage is a special type provided by the encoding/json package that allows you to store arbitrary JSON data without losing type information.

type Message struct {
    Cmd    string
    Data   json.RawMessage
}

Next, define a struct type for each variant of the Data field, in this case, CreateMessage:

type CreateMessage struct {
    Conf map[string]int
    Info map[string]int
}

Now, in your unmarshaling code, you can switch on the value of the Cmd field and decode the Data to the appropriate concrete type:

switch m.Cmd {
case "create":
    var cm CreateMessage
    if err := json.Unmarshal(m.Data, &cm); err != nil {
        log.Fatal(err)
    }
    fmt.Println(m.Cmd, cm.Conf, cm.Info)
default:
    log.Fatal("bad command")
}

This approach allows you to unmarshal JSON into an interface{} while preserving the ability to later convert it to a specific type based on a known value.

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