不支持的扫描：SQL UUID数组

Question

I am trying to get an array of UUID from a PostgreSQL DB, this gives the following error:

sql: Scan error on column index 0: unsupported Scan, storing driver.Value type []uint8 into type *[]string

Fetching a single UUID is no problem, but when it is an array, the Scan function infers the type of the elements as uint8.

Is there a workaround / solution for this? Or should I rethink my DB?

Code :

func FetchListIdsForUser(id string, db *sql.DB) ([]string, error) {

        // where the results will be stored

        var (
            lists []string
        )


        // statement prep

        stmt, err := db.Prepare("select lists from users where object_id = $1")
            if err != nil {
            formattedError := er.New("FetchListIdsForUser SQL Select: " + err.Error())
            log.Println(formattedError)
            return nil,formattedError
        }
        defer stmt.Close()


        // query happening

        rows, err := stmt.Query(id)
        if err != nil {
            formattedError := er.New("FetchListIdsForUser SQL Query: " + err.Error())
            log.Println(formattedError)
            return nil,formattedError
        }

        defer rows.Close()


        // for each row

        for rows.Next() {

            // scan : this is where the error happens.

            err := rows.Scan(&lists)
            if err != nil {
                formattedError := er.New("FetchListIdsForUser SQL Scan: " + err.Error())
                log.Println(formattedError)
                return nil,formattedError
            }
            return lists,nil
        }

        err = rows.Err()
        if err != nil {
            formattedError := er.New("FetchListIdsForUser: " + id + " Does Not Exist")
            log.Println(formattedError)
            return nil,formattedError
        }
        return nil,er.New("FetchListIdsForUser: " + id + " Does Not Exist")
}

Answer 1

Your code can be simplified a lot:

func FetchListIdsForUser(id string, db *sql.DB) ([]string, error) {

        rows, err := db.Query("SELECT unnest(lists) FROM users WHERE object_id = $1", id)
        if err != nil { 
            formattedError := errors.New("FetchListIdsForUser SQL Query: " + err.Error())
            return nil, formattedError
        }

        defer rows.Close()

        var lists []string
        var list string

        for rows.Next() {
            // scan every item of the array and append it to lists
            err := rows.Scan(&list)
            if err != nil {
                formattedError := errors.New("FetchListIdsForUser SQL Scan: " + err.Error())
                return nil,formattedError
            }
            lists = append(lists, list)
        }

        if lists == nil {
            // no rows returned
            formattedError := errors.New("FetchListIdsForUser: " + id + " Does Not Exist")
            return nil, formattedError
        }

        return lists, nil
}

• db.Query() will prepare (and cache) the statement for you. There's no need to do that manually.
• I added unnest(lists) to the select statement. This will produce one row for every element in the array.
• In the for loop, we scan every element individually and append it to lists.

Also, this driver supports scanning arrays out of the box.