added db.QueryRowStruct

Author: ungerik

db/queryrow.go

@@ -0,0 +1,54 @@
+package db
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"reflect"
+
+	"github.com/domonda/go-sqldb"
+)
+
+// QueryRowStruct uses the passed pkValues to query a table row
+// and scan it into a struct of type S that must have tagged fields
+// with primary key flags to identify the primary key column names
+// for the passed pkValues.
+func QueryRowStruct[S any](ctx context.Context, table string, pkValues ...any) (row *S, err error) {
+	if len(pkValues) == 0 {
+		return nil, errors.New("no primaryKeyValues passed")
+	}
+	t := reflect.TypeOf(row).Elem()
+	if t.Kind() != reflect.Struct {
+		return nil, fmt.Errorf("expected struct template type instead of %s", t)
+	}
+	conn := Conn(ctx)
+	pkColumns := pkColumnsOfStruct(t, conn.StructFieldNamer())
+	if len(pkColumns) != len(pkValues) {
+		return nil, fmt.Errorf("got %d primary key values, but struct %s has %d primary key fields", len(pkValues), t, len(pkColumns))
+	}
+	query := fmt.Sprintf(`SELECT * FROM %s WHERE "%s" = $1`, table, pkValues[0])
+	for i := 1; i < len(pkValues); i++ {
+		query += fmt.Sprintf(` AND "%s" = $%d`, pkValues[i], i+1)
+	}
+	err = conn.QueryRow(query, pkValues...).ScanStruct(&row)
+	if err != nil {
+		return nil, err
+	}
+	return row, nil
+}
+
+func pkColumnsOfStruct(t reflect.Type, namer sqldb.StructFieldNamer) (columns []string) {
+	for i := 0; i < t.NumField(); i++ {
+		field := t.Field(i)
+		name, flags, ok := namer.StructFieldName(field)
+		if !ok {
+			continue
+		}
+		if name == "" {
+			columns = append(columns, pkColumnsOfStruct(field.Type, namer)...)
+		} else if flags.PrimaryKey() {
+			columns = append(columns, name)
+		}
+	}
+	return columns
+}