我可以使用MarshalJSON向golang中的json编码添加任意字段吗？

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