MarshalJSON on compound struct doesn’t work as expected [duplicate] - json

Question

This question already has answers here:
Idiomatic way to embed struct with custom MarshalJSON() method
(5 answers)
Closed 27 days ago.
I’m working on an API that receives and sends JSON, nothing fancy here.
The first concern I had was that I needed to only work with UTC times and dates across the API, so any date/time received needed to be converted to UTC.
In order to make this work on already existing structs, I’ve implemented the MarshalJSON() method, and it works like a charm. You can see a simplified example below to illustrate with the Contact struct
package main
import (
"encoding/json"
"fmt"
"time"
)
type Contact struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
CreatedAt time.Time `json:"created_at"`
}
func (c *Contact) MarshalJSON() ([]byte, error) {
type ContactAlias Contact
return json.Marshal(&struct {
*ContactAlias
CreatedAt time.Time `json:"created_at" sql:"created_at"`
}{
ContactAlias: (*ContactAlias)(c),
CreatedAt: c.CreatedAt.UTC(),
})
}
func main() {
contact := &Contact{FirstName: "John", LastName: "Doe", CreatedAt: time.Now()}
s, err := json.Marshal(&contact)
if err != nil {
println(err.Error())
}
fmt.Printf("STEP 1)\n\n%s\n\n\n", s)
}
Then, I needed to add another field to my Contact struct that is Intro
package main
import (
"encoding/json"
"fmt"
"time"
)
type Contact struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
CreatedAt time.Time `json:"created_at"`
}
// STEP-1
func (c *Contact) MarshalJSON() ([]byte, error) {
type ContactAlias Contact
return json.Marshal(&struct {
*ContactAlias
CreatedAt time.Time `json:"created_at" sql:"created_at"`
}{
ContactAlias: (*ContactAlias)(c),
CreatedAt: c.CreatedAt.UTC(),
})
}
// STEP-2
type ContactWithIntro struct {
Contact
Intro string `json:"intro"`
}
func main() {
contact := &Contact{FirstName: "John", LastName: "Doe", CreatedAt: time.Now()}
s, err := json.Marshal(&contact)
if err != nil {
println(err.Error())
}
fmt.Printf("STEP 1)\n\n%s\n\n\n", s)
intro := "Hello World!"
contactWithIntro := ContactWithIntro{
Contact: *contact,
Intro: intro,
}
fmt.Printf("STEP 2-a)\n\n%+v\n\n\n", contactWithIntro)
s, err = json.Marshal(&contactWithIntro)
if err != nil {
println(err.Error())
}
fmt.Printf("STEP 2-b)\n\n%s\n", s)
}
As you can see, I can’t get the intro field in the contactWithInfo JSON string .
After a few tests, I figured out that if I remove the MarshalJSON, then I get the info field in the final JSON but then time is not UTC anymore … I don’t understand what’s happening.
Thank you for your support.
Below is the link to test the snippet if you want to test this out:
https://goplay.tools/snippet/oCBqGT3AR96
Edit 24/01/2023 - undesired workaround
A solution to work with UTC time would be to create a custom type associated to a custom MarshalJSON method but this implies to cast the time value everywhere it's assigned to the field struct. I would prefer avoid this solution.
package main
import (
"encoding/json"
"fmt"
"time"
)
type Contact struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
CreatedAt DateTimeUTC `json:"created_at"`
}
type DateTimeUTC time.Time
func (c DateTimeUTC) MarshalJSON() ([]byte, error) {
return json.Marshal(time.Time(c).UTC().Format(time.RFC3339))
}
func main() {
t := DateTimeUTC(time.Now())
contact := &Contact{FirstName: "John", LastName: "Doe", CreatedAt: t}
s, err := json.Marshal(&contact)
if err != nil {
println(err.Error())
}
fmt.Printf("%s\n", s)
}
snippet here: https://goplay.tools/snippet/V6A11T4z_jy

Answer 1

Because you've embedded the Contact directly the compiler seems to be casting the ContactWithIntro up to Contact, which is why it's ignoring the Intro field.
If you change the declaration:
type ContactWithIntro struct {
Contact Contact
Intro string `json:"intro"`
}
It will then marshal correctly.
Also, at the beginning, you don't need to store the pointer to Contact and then pass a pointer to the pointer to json.Marshal:
contact := Contact{FirstName: "John", LastName: "Doe", CreatedAt: time.Now()} // No & here
...
contactWithIntro := ContactWithIntro{
Contact: contact, // No * here
Intro: intro,
}
This form is actually a bit cleaner (IMHO)
I've amended your playground: https://goplay.tools/snippet/G4CCz99Cjqx