How could I create a new struct from an unpredictable db.Query()? - postgresql

Question

I'm using SELECT * in a db.query() to return columns from a table. Typically, I would fmt.Scan() the rows into a pre-declared struct{} for further manipulation, but in this case, the table columns change frequently so I'm not able to use a declared struct{} as part of my Scan().
I've been struggling to figure out how I might dynamically build a struct{} based on the column results of the db.query() which I could subsequently call on the use of for Scan(). I've read a little about reflect but I'm struggling to determine if this is right for my use-case or if I might have to think about something else.
Any pointers would be greatly appreciated.

Answer 1

you can get column names from resulting rowset and prepare a slice for the scan.
Example (https://go.dev/play/p/ilYmEIWBG5S) :
package main
import (
"database/sql"
"fmt"
"log"
"github.com/DATA-DOG/go-sqlmock"
)
func main() {
// mock db
db, mock, err := sqlmock.New()
if err != nil {
log.Fatal(err)
}
columns := []string{"id", "status"}
mock.ExpectQuery("SELECT \\* FROM table").
WillReturnRows(sqlmock.NewRows(columns).AddRow(1, "ok"))
// actual code
rows, err := db.Query("SELECT * FROM table")
if err != nil {
log.Fatal(err)
}
cols, err := rows.Columns()
if err != nil {
log.Fatal(err)
}
data := make([]interface{}, len(cols))
strs := make([]sql.NullString, len(cols))
for i := range data {
data[i] = &strs[i]
}
for rows.Next() {
if err := rows.Scan(data...); err != nil {
log.Fatal(err)
}
for i, d := range data {
fmt.Printf("%s = %+v\n", cols[i], d)
}
}
}
This example reads all columns into strings. To detect column type one can use rows.ColumnTypes method.