_mapper () {
    const mapper = this.constructor.mapper
    if (!mapper) {
      throw utils.err(`${DOMAIN}#_mapper`, '')(404, 'mapper')
    }
    return mapper
  }
Returns the {@link Mapper} paired with this record's class, if any.

@method Record#_mapper
@returns {Mapper} The {@link Mapper} paired with this record's class, if any.
@since 3.0.0