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.
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
Marshalling a map is another way around the problem.
tmap := make(map[string]interface{})
tmap["struct"] = struct {
StructValue string `json:"struct_value"`
}{
"Value 02",
}
tmap["string"] = "Value 01"
out, err := json.MarshalIndent(tmap, "", " ")
if err != nil {
log.Fatalln(err)
}
fmt.Println(string(out))
This will output:
{
"string": "Value 01",
"struct": {
"struct_value": "Value 02"
}
}
Where you have many arbitrary key names this could be a good solution.
Playground: https://play.golang.org/p/Umy9rtx2Ms
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",
})
}