Golang JSON omitempty With time.Time Field

Question

Trying to json Marshal a struct that contains 2 time fields. But I only want the field to come through if it has a time value. So I'm using json:",omitempty" but it's not working.

What can I set the Date value to so json.Marshal will treat it like an empty (zero) value and not include it in the json string?

Playground: http://play.golang.org/p/QJwh7yBJlo

Actual Outcome:

{"Timestamp":"2015-09-18T00:00:00Z","Date":"0001-01-01T00:00:00Z"}

Desired Outcome:

{"Timestamp":"2015-09-18T00:00:00Z"}

Code:

package main

import (
    "encoding/json"
    "fmt"
    "time"
)

type MyStruct struct {
    Timestamp time.Time `json:",omitempty"`
    Date      time.Time `json:",omitempty"`
    Field     string    `json:",omitempty"`
}

func main() {
    ms := MyStruct{
        Timestamp: time.Date(2015, 9, 18, 0, 0, 0, 0, time.UTC),
        Field:     "",
    }

    bb, err := json.Marshal(ms)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(bb))
}

Answer 1

The omitempty tag option does not work with time.Time as it is a struct. There is a "zero" value for structs, but that is a struct value where all fields have their zero values. This is a "valid" value, so it is not treated as "empty".

But by simply changing it to a pointer: *time.Time, it will work (nil pointers are treated as "empty" for json marshaling/unmarshaling). So no need to write custom Marshaler in this case:

type MyStruct struct {
    Timestamp *time.Time `json:",omitempty"`
    Date      *time.Time `json:",omitempty"`
    Field     string     `json:",omitempty"`
}

Using it:

ts := time.Date(2015, 9, 18, 0, 0, 0, 0, time.UTC)
ms := MyStruct{
    Timestamp: &ts,
    Field:     "",
}

Output (as desired):

{"Timestamp":"2015-09-18T00:00:00Z"}

Try it on the Go Playground.

If you can't or don't want to change it to a pointer, you can still achieve what you want by implementing a custom Marshaler and Unmarshaler. If you do so, you can use the Time.IsZero() method to decide if a time.Time value is the zero value.

Answer 2

you may define you self Time type for custom marshal format, and use it everywhere instead time.Time

http://play.golang.org/p/S9VIWNAaVS

package main

import "fmt"
import "time"
import "encoding/json"

type MyTime struct{
    *time.Time
}

func (t MyTime) MarshalJSON() ([]byte, error) {
        return []byte(t.Format("\"2006-01-02T15:04:05Z\"")), nil
   }


   // UnmarshalJSON implements the json.Unmarshaler interface.
    // The time is expected to be a quoted string in RFC 3339 format.
func (t *MyTime) UnmarshalJSON(data []byte) (err error) {
        // Fractional seconds are handled implicitly by Parse.
        tt, err := time.Parse("\"2006-01-02T15:04:05Z\"", string(data))
        *t = MyTime{&tt}
        return
   }

func main() {
    t := time.Now()
    d, err := json.Marshal(MyTime{&t})
    fmt.Println(string(d), err)
    var mt MyTime
    json.Unmarshal(d, &mt)
    fmt.Println(mt)
}