equals method
The mapping method to compare value and other for equality.
Implementation
@override
bool equals(T value, T other, MappingContext context) {
  if (value.runtimeType.base != other.runtimeType.base) {
    return false;
  }
  return _members.every((f) {
    return context.container.isEqual(f.get(value), f.get(other));
  });
}