Proposed Rows.ScanRow implementation (cut-down version)

scan.go

package model

import (
	"database/sql"
	"errors"
	"fmt"
	"reflect"
	"sync"
)

// ScanRow is a cut-down version of the proposed Rows.ScanRow method. It
// currently only handles dest being a (pointer to) struct, and does not
// handle embedded fields. See https://github.com/golang/go/issues/61637
func ScanRow(rows *sql.Rows, dest any) error {
	rv := reflect.ValueOf(dest)
	if rv.Kind() != reflect.Pointer || rv.IsNil() {
		return errors.New("dest must be a non-nil pointer")
	}

	elem := rv.Elem()
	if elem.Kind() != reflect.Struct {
		return errors.New("dest must point to a struct")
	}
	indexes := cachedFieldIndexes(reflect.TypeOf(dest).Elem())

	columns, err := rows.Columns()
	if err != nil {
		return fmt.Errorf("cannot fetch columns: %w", err)
	}

	var scanArgs []any
	for _, column := range columns {
		index, ok := indexes[column]
		if ok {
			// We have a column to field mapping, scan the value.
			field := elem.Field(index)
			scanArgs = append(scanArgs, field.Addr().Interface())
		} else {
			// Unassigned column, throw away the scanned value.
			var throwAway any
			scanArgs = append(scanArgs, &throwAway)
		}
	}
	return rows.Scan(scanArgs...)
}

// fieldIndexes returns a map of database column name to struct field index.
func fieldIndexes(structType reflect.Type) map[string]int {
	indexes := make(map[string]int)
	for i := 0; i < structType.NumField(); i++ {
		field := structType.Field(i)
		tag := field.Tag.Get("sql")
		if tag != "" {
			// Use "sql" tag if set
			indexes[tag] = i
		} else {
			// Otherwise use field name (with exact case)
			indexes[field.Name] = i
		}
	}
	return indexes
}

var fieldIndexesCache sync.Map // map[reflect.Type]map[string]int

// cachedFieldIndexes is like fieldIndexes, but cached per struct type.
func cachedFieldIndexes(structType reflect.Type) map[string]int {
	if f, ok := fieldIndexesCache.Load(structType); ok {
		return f.(map[string]int)
	}
	indexes := fieldIndexes(structType)
	fieldIndexesCache.Store(structType, indexes)
	return indexes
}