json.RawMessage from database/sql json column getting overwritten - sql

Question

Getting strange behaviour with a struct with embedded json.
package main
import (
"database/sql"
"encoding/json"
"fmt"
_ "github.com/lib/pq"
)
type Article struct {
Id int
Doc *json.RawMessage
}
func main() {
db, err := sql.Open("postgres", "postgres://localhost/json_test?sslmode=disable")
if err != nil {
panic(err)
}
_, err = db.Query(`create table if not exists articles (id serial primary key, doc json)`)
if err != nil {
panic(err)
}
_, err = db.Query(`truncate articles`)
if err != nil {
panic(err)
}
docs := []string{
`{"type":"event1"}`,
`{"type":"event2"}`,
}
for _, doc := range docs {
_, err = db.Query(`insert into articles ("doc") values ($1)`, doc)
if err != nil {
panic(err)
}
}
rows, err := db.Query(`select id, doc from articles`)
if err != nil {
panic(err)
}
articles := make([]Article, 0)
for rows.Next() {
var a Article
err := rows.Scan(
&a.Id,
&a.Doc,
)
if err != nil {
panic(err)
}
articles = append(articles, a)
fmt.Println("scan", string(*a.Doc), len(*a.Doc))
}
fmt.Println()
for _, a := range articles {
fmt.Println("loop", string(*a.Doc), len(*a.Doc))
}
}
Output:
scan {"type":"event1"} 17
scan {"type":"event2"} 17
loop {"type":"event2"} 17
loop {"type":"event2"} 17
So the articles end up pointing to the same json.
Am I doing something wrong?
UPDATE
Edited to a runnable example. I'm using Postgres and lib/pq.

Answer 1

I ran into this same issue and after looking at if for a long time I read the doc on Scan and it says
If an argument has type *[]byte, Scan saves in that argument a copy of the corresponding data. The copy is owned by the caller and can be modified and held indefinitely. The copy can be avoided by using an argument of type *RawBytes instead; see the documentation for RawBytes for restrictions on its use.
What I think is happening if you use *json.RawMessage then Scan does not see it as a *[]byte and does not copy into it. So you get in internal slice on the next loop Scan overwrites.
Change your Scan to cast the *json.RawMessage to a *[]byte so Scan will copy the values to it.
err := rows.Scan(
&a.Id,
(*[]byte)(a.Doc),
)

Answer 2

In case that helps anyone :
I used masebase anwser to INSERT a json.RawMessage property of my struct in a postgresql db column having jsonb column type.
All you need to do is cast : ([]byte)(a.Doc) in the insert binding method (without the * in my case).