encoding/json: Marshaler interface not producing "correct"/desired results

[pjebs]

type AWorker struct {
	ID        string  `json:"id"`
	Name      *string `json:"name"`
	AvatarURL *string `json:"avatar_url"`
}

type ReqUser struct {
	ExpireAt *time.Time `json:"expire_at"`
	Status   string     `json:"status"`
	Notes    *string    `json:"notes"`
}

func (r ReqUser) MarshalJSON() ([]byte, error) {
	out := map[string]interface{}{}
	out["status"] = r.Status
	out["notes"] = r.Notes
	if r.ExpireAt != nil {
		out["expire_at"] = (*r.ExpireAt).Format("2006-01-02")
	}
	return json.Marshal(out)
}

type AUser struct {
	AWorker
	ReqUser
}

[pjebs]

I want to json/Marshall a []AUser.

When I remove func (r ReqUser) MarshalJSON():

"users": [
    {
      "id": "PJ_WORKER1",
      "name": "PJ 1",
      "avatar_url": null,
      "expire_at": null,
      "status": "completed",
      "notes": null
    },
    {
      "id": "PJ_WORKER2",
      "name": "PJ 2",
      "avatar_url": null,
      "expire_at": "2020-12-31T00:00:00Z",
      "status": "incomplete",
      "notes": null
    },
    {
      "id": "PJ_WORKER3",
      "name": "PJ 3",
      "avatar_url": null,
      "expire_at": "2018-12-31T00:00:00Z",
      "status": "completed",
      "notes": "id looks fake"
    }
  ]

[pjebs]

When I keep func (r ReqUser) MarshalJSON():

"users": [
    {
      "expire_at": null,
      "notes": null,
      "status": "completed"
    },
    {
      "expire_at": "2020-12-31",
      "notes": null,
      "status": "incomplete"
    },
    {
      "expire_at": "2018-12-31",
      "notes": "id looks fake",
      "status": "completed"
    }
  ]

[pjebs]

if this is technically correct behaviour, then perhaps for Go2, it should be changed to produce "desired" results.

[dmitshur]

Which one is the desired or expected result?

[pjebs]

AUser has 2 structs embedded. It should display json fields from AWorker and ReqUser, but for ReqUser it should produce the results from the MarshalJSON() function (i.e. the corrected date format)

[dmitshur]

I see. Here's a runnable version on the playground: https://play.golang.org/p/FUBH0ZlswaL.

What's happening is ReqUser struct is being embedded into AUser, and as a result, AUser implements the json.Marshaler interface via ReqUser.MarshalJSON method. That way, all the AWorker fields get left out when AUser gets JSON marshaled.

It's working correctly given the rules of struct field embedding and json.Marshaler interface, but I can see how the two features are not interacting in an orthogonal and helpful way. I'm not sure how this can be improved, but perhaps other people will have ideas.

[dsnet]

Given a struct type T with an embedded type R, it is impossible to determine whether T.Method is actually explicitly defined on T or is forwarded from R. The forwarding of methods in addition to fields is not really a gotcha of the json package but embedding in general.

If your intention to to forward only the fields, but not the methods, you can do this:

type aWorker AWorker
type reqUser ReqUser

type AUser struct {
	aWorker
	reqUser
}

Declaring aWorker and reqUser creates two new struct types that have the exact same layout as AWorker and ReqUser, but none of the methods.

I'm closing this as a duplicate of #22013.

[pjebs]

@dsnet I require the methods to be attached, because encoding/json requires the custom MarshalJSON() function so I can modify the marshalled output.

[dsnet]

What you want is currently not possible since you want to forward the fields of ReqUser, but don't want to forward the methods. At the same time you don't want to remove the ReqUser.MarshalJSON method.

Either way, this is still a language issue with embedding and #22013 is the place to rethink embedding for Go2.

[dsnet]

I should also note that this is related to #6213, which may be the reason why you are embedding in the first place.