code: func mapper() *reflectx.Mapper {
  	mprMu.Lock()
  	defer mprMu.Unlock()
  
  	if mpr == nil {
  		mpr = reflectx.NewMapperFunc("db", NameMapper)
  	} else if origMapper != reflect.ValueOf(NameMapper) {
  		// if NameMapper has changed, create a new mapper
  		mpr = reflectx.NewMapperFunc("db", NameMapper)
  		origMapper = reflect.ValueOf(NameMapper)
  	}
  	return mpr
  }
docs: // mapper returns a valid mapper using the configured NameMapper func.