External object linked through foreign key in hibernate and MySql

Question

I'm using Spring data with Hibernate and MySql and I have a doubt. My entity is

@Entity
@Table(name = "car", catalog = "DEMO")
public class Car implements java.io.Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    private Integer idCar;
    @JsonBackReference
    private CarType carType;
    @JsonBackReference
    private Fleet fleet;
    private String id;
    private int initialKm;
    private String carChassis;
    private String note;
    @JsonManagedReference
    private Set<Acquisition> acquisitions = new HashSet<Acquisition>(0);

with get and set method. Sometimes, I need external object as carType, another entity. If I use this webservice

@Override
    @RequestMapping(value = { "/cars/{idFleet}"}, method = RequestMethod.GET)
    public String getCars(@PathVariable int idFleet, Model model){  
        try{
            model.addAttribute("carsList",fleetAndCarService.findCarsByIdFleet(idFleet));
            //Modal parameter
            model.addAttribute("carTypeList",fleetAndCarService.getCarsType());
            model.addAttribute("fleetApplication",fleetAndCarService.getFleetById(idFleet));            
            model.addAttribute("carForm", new CarForm());
            model.addAttribute("error",false);
        }catch (Exception e){
            LOG.error("Threw exception in FleetAndCarControllerImpl::getCars : " + ErrorExceptionBuilder.buildErrorResponse(e));
            model.addAttribute("error",true);
        }
        return "cars";
    }

from my html page I can retrieve carType.idCarType,but if I use this

@Override
    @RequestMapping(value = { "/cars/{idFleet}"}, method = RequestMethod.GET)
    public @ResponseBody TableUI getCars(@PathVariable int idFleet) {   
        TableUI ajaxCall=new TableUI();
        try {   
            ajaxCall.setData(fleetAndCarService.findCarsByIdFleet(idFleet));
            return ajaxCall;
        } catch (QueryException e) {
            ErrorResponse errorResponse= ErrorResponseBuilder.buildErrorResponse(e);
            LOG.error("Threw exception in FleetAndCarControllerImpl::addCar :" + errorResponse.getStacktrace());
            return ajaxCall;
        }
    }

where TableUi has only a field data where I put the result to use it into datatables, I don't have carType and fleet. Why? Do I have to use Hibernate.initialize, and how so it is a list?Thansk,regards

Also this update doesn't work:

@Override
@Transactional
public List<Car> findByFleetIdFleet(int idFleet) {
    List<Car> carList= carRepository.findByFleetIdFleet(idFleet);
    for (Car car:carList)
        Hibernate.initialize(car.getCarType());
    return carList; 
}

Answer 1

You could call Hibernate.initialize on each element

Collection<Car> cars = fleetAndCarService.findCarsByIdFleet(idFleet);
for(Car car : cars) {
    Hibernate.initialize(car.getCarType());
    Hibernate.initialize(car.getFleet());
}
ajaxCall.setData();
return ajaxCall;

This would be a good starting point and would allow you to move forwards. At high scales however this could become a performance bottleneck as it will perform a query with each call to initialize so you will have 2*n queries to the database.

For maximum performance you will have several other options:

1. Iterate through the cars and build up a list of IDs and then query for the car types by ID in a single query with the list of IDs. Do the same for the fleets. Then call Hibernate.initialize. The first two queries will populate the persistence context and the call to initialize will not need to go to the database.

2. Create a special query for this call which fetch joins the properties you will need.

3. Setup batch fetching which will fetch the cards and fleets in batches instead of one car/fleet per query.

4. Use a second level cache so the initialization causes Hibernate to pull from the cache instead of the database.

Describing these options in details is beyond the scope of a single question but a good place to start would be Hibernate's documentation on performance.