encoding/json: json.RawMessage does not works correctly

[gromas]

I want to unmarshal JSON to different struct types and I write:

	bytes := []byte(`
		{"__type":"instance.started","InstanceId":"a9ff34dd-d20b-4887-99d6-f2f3a68f86a5"}
	`)
	var envelope struct {
		Type string `json:"__type"`
		Json *json.RawMessage
	}

        _ = json.Unmarshal(bytes, &envelope)

	fmt.Println(envelope.Type)
	fmt.Println(envelope.Json)

I get empty envelope.Json but must be an byte array (json.RawMessage).

repro:
https://play.golang.org/p/m6wd44R35FZ

[ALTree]

Hi,

matching is based on field names and annotations.

From the Json and Go post:

For a given JSON key "Foo", Unmarshal will look through the destination struct's fields to find (in order of preference):

• An exported field with a tag of "Foo" (see the Go spec for more on struct tags),
• An exported field named "Foo", or
• An exported field named "FOO" or "FoO" or some other case-insensitive match of "Foo".

So you need to change your struct to either

	var envelope struct {
		Type string `json:"__type"`
		InstanceId *json.RawMessage
	}

or

	var envelope struct {
		Type string `json:"__type"`
		Json *json.RawMessage `json:"InstanceId"`
	}

Closing here, since this is not a bug.

[gromas]

Hey! Thank you for answer. InstanceId is sample field. I have over 100 different message types and want to reflect json to different golang structs (I use switch evnvelope.Type ... json.Unmarshal(envelope.Json, &concreteType)...) I don't want json:"InstanceId"

And this is a bug with json.RawMessage

[ALTree]

EDIT: yeah misread. Re-opening.

[gromas]

Same problem with marshaling. I marshal {Type, some_message} data and get some_message only without __type ))

https://play.golang.org/p/SsgQmtq2yP0

[toothrot]

Hi @gromas,

Referring to your first example:

	bytes := []byte(`
		{"__type":"instance.started","InstanceId":"a9ff34dd-d20b-4887-99d6-f2f3a68f86a5"}
	`)
	
	var envelope struct {
		Type string `json:"__type"`
		Json *json.RawMessage
	}

What exactly do you expect to appear in the Json struct field?

[seankhliao]

@gromas
The go struct structure has to match the json structure, you cannot just dump all the unrecognized fields into a json.RawMessage
What you can do is unmarshal twice, once to find the type, second time into a known struct. json.RawMessage is for making this more convenient in more deeply nested structs

Your second example you are embedding a json.RawMessage whose MarshalJSON/UnmarshalJSON is being promoted to envelope and overriding the marshal process

[toothrot]

Thanks @seankhliao. I'm going to close this issue, as it is not clear that this is a bug. @gromas, please see one of the resources on https://golang.org/wiki/Questions for further help using the encoding/json package.