Jpa 在JPQL查询中使用COUNT

我有以下JPQL查询：

    List<DestinationInfo> destinations = em.createQuery("SELECT NEW com.realdolmen.patuva.dto.DestinationInfo(d.name, d.continent, MIN(t.departureDate), MIN(t.pricePerDay), COUNT(t.id))" +
            " FROM Destination d, Trip t" +
            " WHERE d.continent = :continent " +
            " GROUP BY d.name, d.continent").setParameter("continent", searchedContinent).getResultList();

如果我省略了

COUNT（t.id）

并从

DestinationInfo

构造函数中删除该参数，它就可以正常工作。为什么我不能将

计数（t.id）

映射到我的

DestinationInfo

DTO

这是我的

DestinationInfo

课程：

public class DestinationInfo {
    private String name;
    private Continent continent;
    private Date earliestDeparture;
    private Integer totalNumTrips;
    private BigDecimal lowestPrice;

    public DestinationInfo(String name, Continent continent, Date earliestDeparture, BigDecimal lowestPrice, Integer totalNumTrips) {
        this.name = name;
        this.continent = continent;
        this.earliestDeparture = earliestDeparture;
        this.totalNumTrips = totalNumTrips;
        this.lowestPrice = lowestPrice;
    }

    // getters and setters
}

显然

COUNT（t.id）

返回一个类型为

long

的数字。将

DestinationInfo

类更改为以下内容可使其正常工作：

public class DestinationInfo {
    private String name;
    private Continent continent;
    private Date earliestDeparture;
    private long totalNumTrips;
    private BigDecimal lowestPrice;

    public DestinationInfo(String name, Continent continent, Date earliestDeparture, BigDecimal lowestPrice, long totalNumTrips) {
        this.name = name;
        this.continent = continent;
        this.earliestDeparture = earliestDeparture;
        this.totalNumTrips = totalNumTrips;
        this.lowestPrice = lowestPrice;
    }

    // getters and setters
}

---

@AndreiI yep，我最初把它设为

int

，然后把它改为

Integer

，看看这是否行得通

public class DestinationInfo {
    private String name;
    private Continent continent;
    private Date earliestDeparture;
    private long totalNumTrips;
    private BigDecimal lowestPrice;

    public DestinationInfo(String name, Continent continent, Date earliestDeparture, BigDecimal lowestPrice, long totalNumTrips) {
        this.name = name;
        this.continent = continent;
        this.earliestDeparture = earliestDeparture;
        this.totalNumTrips = totalNumTrips;
        this.lowestPrice = lowestPrice;
    }

    // getters and setters
}