Can I use MarshalJSON to add arbitrary fields to a json encoding in golang?

Question

Suppose I've written the following code snippet. Full code on the playground here for those inclined.

type Book struct {   Title        string   Author       string }  func main() {   ms := Book{"Catch-22", "Joseph Heller"}   out, err := json.MarshalIndent(ms, "", "  ")   if err != nil {     log.Fatalln(err)   }   fmt.Println(string(out)) } 

This code outputs the following, exactly as I'd expect:

{   "Title": "Catch-22",   "Author": "Joseph Heller" } 

Suppose for a moment I wanted to add a field to the JSON output without including it in the Book struct. Perhaps a genre:

{   "Title": "Catch-22",   "Author": "Joseph Heller",   "Genre": "Satire" } 

Can I use MarshalJSON() to add an arbitrary field to the JSON payload on Marshal()? Something like:

func (b *Book) MarshalJSON() ([]byte, error) {     // some code } 

Other answers make me think this should be possible, but I'm struggling to figure out the implementation.

Answer 1

Here's a better answer than my previous one.

type FakeBook Book  func (b Book) MarshalJSON() ([]byte, error) {     return json.Marshal(struct {         FakeBook         Genre string     }{         FakeBook: FakeBook(b),         Genre:    "Satire",     }) } 

Since anonymous struct fields are "merged" (with a few additional considerations) we can use that to avoid remapping the individual fields. Note the use of the FakeBook type to avoid the infinite recursion which would otherwise occur.

Playground: http://play.golang.org/p/21YXhB6OyC

Answer 2

One possible answer to this question is a struct literal (code here), although I'm hoping for something a bit more general, which doesn't require remapping all of the struct's fields:

func (b *Book) MarshalJSON() ([]byte, error) {     return json.Marshal(struct{         Title    string         Author   string         Genre    string     } {         Title: b.Title,         Author: b.Author,         Genre: "Satire",     }) }