How to Unmarshal JSON into Go Interfaces Based on a Dynamic Field Value?

Unmarshalling JSON into Interfaces in Go

Consider the scenario where you have a JSON string with varied data structures depending on a specific field, such as the "cmd" field. You want to unmarshal this JSON into a Go struct containing a field with an interface{} type and adapt it to different data structures based on the "cmd" value.

Solution

To achieve this, define a base struct with fixed fields and a json.RawMessage field to hold the variant data. Then, create specific struct types for each variant and unmarshal to them based on the command.

For example:

type Message struct {
  Cmd string `json:"cmd"`
  Data      json.RawMessage
}  

type CreateMessage struct {
    Conf map[string]int `json:"conf"`
    Info map[string]int `json:"info"`
}  

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

In this example, the Message struct represents the overall message structure, containing the "cmd" field and a json.RawMessage field to hold the variant data. CreateMessage is a specific struct type representing the "create" command variant.

The code unmarshals the initial JSON string into a Message variable and then uses the "cmd" field to determine which specific variant to unmarshal and print.

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