Data Mapping Layer

@Mapper(componentModel = "spring")
public interface SupplierMapper {
    
    // Entity → DTO
    SupplierDTO toDTO(Supplier supplier);
    
    // DTO → Entity
    Supplier toEntity(CreateSupplierDTO dto);
    
    // List transformation
    List<SupplierDTO> toDTOList(List<Supplier> suppliers);
    
    @Mapping(target = "id", ignore = true)
    @Mapping(target = "createdAt", ignore = true)
    @Mapping(target = "createdBy", ignore = true)
    void update(UpdateSupplierDTO dto, @MappingTarget Supplier supplier);
}

@Component
public class InventoryItemMapper {
    
    public InventoryItemDTO toDTO(InventoryItem entity) {
        if (entity == null) return null;
        
        return InventoryItemDTO.builder()
            .id(entity.getId())
            .name(entity.getName())
            .sku(entity.getSku())
            .supplierId(entity.getSupplierId())
            .supplierName(entity.getSupplier().getName())
            .quantity(entity.getQuantity())
            .unitPrice(entity.getUnitPrice())
            .totalValue(entity.getQuantity() * 
                entity.getUnitPrice().doubleValue())
            .createdAt(entity.getCreatedAt())
            .createdBy(entity.getCreatedBy())
            .build();
    }
    
    public InventoryItem toEntity(CreateInventoryItemDTO dto) {
        if (dto == null) return null;
        
        return InventoryItem.builder()
            .id(UUID.randomUUID().toString())
            .name(dto.getName())
            .sku(dto.getSku())
            .supplierId(dto.getSupplierId())
            .quantity(dto.getInitialQuantity())
            .unitPrice(dto.getUnitPrice())
            .build();
    }
}