How do I inline fields when marshalling JSON?

Question

I have a type MonthYear defined like

type MonthYear time.Time

func (my *MonthYear) MarshalJSON() ([]byte, error) {
    t := time.Time(*my)

    return json.Marshal(&struct {
        Month int `json:"month"`
        Year  int `json:"year"`
    }{
        Month: int(t.Month()) - 1,
        Year:  t.Year(),
    })
}

I'm including it a lot of different structs like

type Event struct {
    Name string `json:"name"`
    Date MonthYear
}

type Item struct {
    Category string `json:"category"`
    Date     MonthYear
}

How do I inline the MonthYear type so that the resulting JSON does not have any embedded objects?

I want the result to look like { "name": "party", "month": 2, "year": 2017 } and { "category": "art", "month": 3, "year": 2016 } without having to write the MarshalJSON for each of the structs.

Answer 1

I know this is not the answer you wish to receive, but until inline support is added to the encoding/json package, you may use the following workaround:

Make your MonthYear a struct, e.g.:

type MonthYear struct {
    t     time.Time
    Month int `json:"month"`
    Year  int `json:"year"`
}

An optional, constructor function for easy creation:

func NewMonthYear(t time.Time) MonthYear {
    return MonthYear{
        t:     t,
        Month: int(t.Month()) - 1,
        Year:  t.Year(),
    }
}

And use embedding (anonymous field) instead of a regular (named) field to get "flattened" / inlined in the JSON representation:

type Event struct {
    Name string `json:"name"`
    MonthYear
}

type Item struct {
    Category string `json:"category"`
    MonthYear
}

As an extra, you'll be able to refer to fields directly like Event.Year or Event.Month which is nice.

Testing it:

evt := Event{Name: "party", MonthYear: NewMonthYear(time.Now())}
fmt.Println(json.NewEncoder(os.Stdout).Encode(evt), evt.Year)

itm := Item{Category: "Tool", MonthYear: NewMonthYear(time.Now())}
fmt.Println(json.NewEncoder(os.Stdout).Encode(itm))

Output (try it on the Go Playground):

{"name":"party","month":10,"year":2009}
<nil> 2009
{"category":"Tool","month":10,"year":2009}
<nil>

Note: The MonthYear.t time field does not play a role here (it is not marshaled either). You can remove it if the original time.Time is not needed.