Create heterogeneous json array in go

Question

Suppose I have an struct like this in go:

type Message struct {
    Args   []interface{}
    Kwargs map[string]interface{}
}

message := Message{                                                                                                                                                                                            
    []interface{}{1, 2, 3, 4},                                                                                                                                                                                 
    map[string]interface{}{"a": 2, "b": 3},                                                                                                                                                                    
}

How should I marshal message to have a JSON like this?

[[1,2,3,4], {"a": 2, "b":3}]

Marshal this value: []interface{}{message.Args, message.Kwargs} Is this good enough for you? — icza
– icza, Commented Nov 19, 2018 at 12:23

klvmungai · Accepted Answer · 2018-11-19 15:28:26Z

2

You can add a marshal method to your struct to handle the logic. Something in the lines of

func (m Message) MarshalJSON() ([]byte, error) {
    data := make([]interface{}, 0)
    data = append(data, m.Args)
    data = append(data, m.Kwargs)
    return json.Marshal(data)
}

Try it on the Playground

edited Nov 19, 2018 at 15:28

answered Nov 19, 2018 at 12:34

klvmungai

8241 gold badge13 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

icza · Accepted Answer · 2018-11-19 13:11:26Z

What you want in your output is a JSON array holding the Args and Kwargs fields of your message struct value, so you may get what you want by marshaling the following slice value:

[]interface{}{message.Args, message.Kwargs}

For example:

message := Message{
    []interface{}{1, 2, 3, 4},
    map[string]interface{}{"a": 2, "b": 3},
}

err := json.NewEncoder(os.Stdout).
    Encode([]interface{}{message.Args, message.Kwargs})

fmt.Println(err)

Output of the above (try it on the Go Playground):

[[1,2,3,4],{"a":2,"b":3}]
<nil>

This works for this specific case. If you want a general solution which marshals fields of a struct value like elements of an array, you may create a helper function that "packs" fields into a slice:

func getFields(i interface{}) (res []interface{}) {
    v := reflect.ValueOf(i)
    if v.Kind() == reflect.Ptr {
        v = v.Elem()
    }
    if v.Kind() != reflect.Struct {
        return nil
    }

    for i := 0; i < v.NumField(); i++ {
        f := v.Field(i)
        if f.CanInterface() {
            res = append(res, f.Interface())
        }
    }
    return res
}

The above getFields() accepts struct values and pointers to structs. Example using it:

message := Message{
    []interface{}{1, 2, 3, 4},
    map[string]interface{}{"a": 2, "b": 3},
}

err := json.NewEncoder(os.Stdout).Encode(getFields(message))
fmt.Println(err)

err = json.NewEncoder(os.Stdout).Encode(getFields(&message))
fmt.Println(err)

Output (try it on the Go Playground):

[[1,2,3,4],{"a":2,"b":3}]
<nil>
[[1,2,3,4],{"a":2,"b":3}]
<nil>

Thanks a lot for your time and answer. I finally found out that using json interfaces (MarshalJson and UnmarshalJson) is a bit more easier.

Collectives™ on Stack Overflow

Create heterogeneous json array in go

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related