I need to loop through returned sql.Rows multiple times. Are my only two options:
In other words, there is no way to go back in sql.Rows (i.e., opposite of Rows.Next).
Another solution would be to use the decorator pattern:
// A RowsDecorator wraps sql.Rows and allows a callback to be called whenever Scan is called
type RowsDecorator struct {
*sql.Rows
OnScan func([]interface{}, error)
}
func Wrap(rows *sql.Rows, onScan func([]interface{}, error)) *RowsDecorator {
return &RowsDecorator{Rows: rows, OnScan: onScan}
}
// Scan calls Rows.Scan and an optional callback
func (rows *RowsDecorator) Scan(dest ...interface{}) error {
err := rows.Rows.Scan(dest...)
if rows.OnScan != nil {
rows.OnScan(dest, err)
}
return err
}
Used like this:
db.Exec(`CREATE TABLE example (id INTEGER, txt TEXT)`)
db.Exec(`INSERT INTO example (id, txt) VALUES (1, 'test-1'), (2, 'test-2'), (3, 'test-3') `)
rawrows, err := db.Query("SELECT id, txt FROM example")
if err != nil {
log.Fatal(err)
}
defer rawrows.Close()
sum := 0
rows := Wrap(rawrows, func(dest []interface{}, err error) {
if err == nil {
sum += *dest[0].(*int)
}
})
for rows.Next() {
var id int
var txt string
err := rows.Scan(&id, &txt)
if err != nil {
log.Fatal(err)
}
log.Println(id, txt)
}
log.Println("sum", sum)
With this pattern you can write a custom function that is called as you iterate over the collection. By using an unnamed, embedded type all of the original methods (Next, Close, etc) can still be called.
