Question

我有3张表格，如：

车

carid|carname|carPrice|
100|bmw|10L|
200|honda|5L|
..
..

carType

carTypeid|Type|
1|suv|
2|xuv|
3|sedan|

carconfig

carid|carTypeid|
100|3|
200|3|

我有3个表的实体。

下面我粘贴了加入表 carconfig ：

的代码

@Entity
@Table(name = "CARCONFIG", uniqueConstraints = { 
        @UniqueConstraint(columnNames = { "carId" } )
})
public class Carconfig {

    @Id
    long carId;

    @OneToOne
    @JoinColumn(name = "carId", insertable = false, updatable = false)
    Car car;

    @ManyToOne(optional = false)
    @JoinColumn(name = "carTypeId")
    CarType carType;

    public Carconfig() {
        super();
    }

    public Carconfig(long carId, CarType carType) {
        super();
        this.carType = carType;
        this.carId = carId;
    }

    public Car getCar() {
        return car;
    }

    public void setCar(Car car) {
        this.car = car;
    }

    public CarType getCarType() {
        return carType;
    }

    public void setCarType(CarType carType) {
        this.carType = carType;
    }

    public long getCarId() {
        return carId;
    }

    public void setCarId(long carId) {
        this.carId = carId;
    }

}

我的问题是：

我想知道如何从表格 car 和 carType 获取数据并将其写入csv。

以下是获取数据的代码。

public String downloadcars(CarForm form, Model model,HttpServletRequest request, HttpServletResponse response)throws IOException {
        PrintWriter pw = null;

        try {
            logger.debug("+++++++++++++++++++++++++++++++++++++++");
            response.setContentType("application/vnd.ms-excel");
            response.setHeader("Content-disposition","attachment; filename=car" + ".csv");
            response.setHeader("Expires", "0");
            response.setHeader("Cache-Control","must-revalidate, post-csheck=0, pre-check=0");
            response.setHeader("Pragma", "public");
            pw = response.getWriter();
            // Get, Write and flush the data.
            pw.println(" Id,Name,Price,Type");
            List<car> carFromDB = Collections.emptyList();



            for (Car car : carFromDB) {             
                pw.println(String.format("%s,%s,%s,%s,%s",
             // this is where i need help

            }
            pw.flush();

        } catch (Exception ex) {
            logger.error("Error while downloading car", ex);
            model.addAttribute("failureMsg","Error while downloading cars");
        }
        return null;
    }

Answer 1

你可以使用 HQL 和Join查询一起完成，如下所示：

List<Object[]> carsWithDetails = session.createQuery(
    "select distinct car.carid, car.carname, car.carPrice, cartype.Type " +
    "from Car  car join car.carType cartype ")
    .list();

然后遍历此list Object[]数组以使用Iterator提取汽车详细信息：

Iterator<Object[]> it = carsWithDetails.iterator();
while(it.hasNext()){
  Object[] car = (Object[]) it.next();
  System.out.println("Car Details:  ");
  System.out.println("Id: "+car[0]);
  System.out.println("Name: "+car[1]);
  System.out.println("Price: "+car[2]);
  System.out.println("Type: "+car[3]);
}

当然，更改循环以满足您的需求，添加代码以写入CSV而不是System.out.println行。

它将取代此循环：

for (Car car : carFromDB) {             
            pw.println(String.format("%s,%s,%s,%s,%s",
         // this is where i need help

        }

修改

如果您想加入所有三个表格，只需在查询中添加加入参考join car.carConfig carConfig，如下所示：

List<Object[]> carsWithDetails = session.createQuery( "select distinct car.carid, car.carname, car.carPrice, cartype.Type " + "from Car car join car.carType cartype join car.carConfig carConfig") .list();

Hibernate - 使用多对一注释从多个表中获取数据

1 个答案: