How do you marshal a sql.NullString such that the output is flattened to give just the value in go?
Here is the code,
package main
import (
"database/sql"
"encoding/json"
"fmt"
"log"
)
//Company details
type Company struct {
ID int `json:"id"`
Abn NullString `json:"abn"`
}
//NullString is a wrapper around sql.NullString
type NullString sql.NullString
//MarshalJSON method is called by json.Marshal,
//whenever it is of type NullString
func (x *NullString) MarshalJSON() ([]byte, error) {
if !x.Valid {
return []byte("null"), nil
}
return json.Marshal(x.String)
}
func main() {
company := &Company{}
company.ID = 68
//create new NullString value
nStr := sql.NullString{String: "hello", Valid: true}
//cast it
company.Abn = NullString(nStr)
result, err := json.Marshal(company)
if err != nil {
log.Println(err)
}
fmt.Println(string(result))
}
Here is the blog post which explains it in detail.
You cannot, at least not using just sql.NullString
and encoding/json
.
What you can do is to declare a custom type that embeds sql.NullString
and have that custom type implement the json.Marshaler
interface.
type MyNullString struct {
sql.NullString
}
func (s MyNullString) MarshalJSON() ([]byte, error) {
if s.Valid {
return json.Marshal(s.String)
}
return []byte(`null`), nil
}
type Company struct {
ID int `json:"id"`
Abn MyNullString `json:"abn,string"`
}
https://play.golang.org/p/Ak_D6QgIzLb
The question suggests that you want to expose your database structure as JSON (presumably REST-ish) API. Unless the project is going to have a short lifespan or the logic layer is trivial, such an approach is considered an antipattern. The internals (database structure) become coupled with the external interface (API) and may result in a high cost of making a change.
I'm attaching some reads as Google is full of tutorials on how to do the opposite:
https://lostechies.com/jimmybogard/2016/05/12/entities-arent-resources-resources-arent-representations/
https://thorben-janssen.com/dont-expose-entities-in-api/