JPA GROUP BY 实体 - 这可能吗？

Question

是否可以通过引用实体分组来选择 JPA 中的数据？

我的意思是：我有两个实体 - 保险和引用的多对一车辆。 保险实体具有 validTill 字段（当然还有车辆字段）。

我想选择车辆，而且是最新的保险。 下面的查询不起作用：

SELECT DISTINCT v.vehicle, 
                max(v.validTill) as lastValidTill 
FROM TraInsurance v 
     GROUP BY v.vehicle 
     ORDER BY lastValidTill

上面的查询失败并出现错误：

ERROR: column "travehicle1_.id_brand" must appear in the GROUP BY clause or be used in an aggregate function

这是因为 JPA 将引用车辆的所有字段添加到查询而不是 GROUP BY。 这是我做错了什么吗？ 或者也许只是不可能做到这一点？

编辑：

TraInsurance实体

@Entity
@Table(name = "TRA_INSURANCES", schema="public")
@SequenceGenerator(name = "TRA_INSURANCES_SEQ", sequenceName = "TRA_INSURANCES_SEQ", allocationSize = 1)
public class TraInsurance implements EntityInt, Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "id", nullable = false)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "TRA_INSURANCES_SEQ")
    private Long                    id;

    @NotNull
    @ManyToOne
    @JoinColumn(nullable = false, name = "id_vehicle")
    private TraVehicle              vehicle;

    @NotNull
    @Column(name = "valid_from", nullable = false)
    private Date                    validFrom;

    @Column(name = "valid_till", nullable = false)
    private Date                    validTill;

    @NotNull
    @ManyToOne
    @JoinColumn(nullable = false, name = "id_company")
    private Company                 company;

    @Column(name = "policy_no", nullable = true, length = 50)
    private String                  policyNumber;

    @Column(name = "rate", nullable = true, precision = 12, scale = 2)
    private BigDecimal              rate;

    @Column(name = "discount_percent", nullable = true)
    private Float                   discountPercent;

    @Column(nullable = true)
    private String                  description;    

    public TraInsurance() {}

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public TraVehicle getVehicle() {
        return vehicle;
    }

    public void setVehicle(TraVehicle vehicle) {
        this.vehicle = vehicle;
    }  

    public Date getValidFrom() {
        return validFrom;
    }

    public void setValidFrom(Date validFrom) {
        this.validFrom = validFrom;
    }

    public Date getValidTill() {
        return validTill;
    }

    public void setValidTill(Date validTill) {
        this.validTill = validTill;
    }

    public Company getCompany() {
        return company;
    }

    public void setCompany(Company company) {
        this.company = company;
    }

    public String getPolicyNumber() {
        return policyNumber;
    }

    public void setPolicyNumber(String policyNumber) {
        this.policyNumber = policyNumber;
    }

    public BigDecimal getRate() {
        return rate;
    }

    public void setRate(BigDecimal rate) {
        this.rate = rate;
    }

    public Float getDiscountPercent() {
        return discountPercent;
    }

    public void setDiscountPercent(Float discountPercent) {
        this.discountPercent = discountPercent;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((id == null) ? 0 : id.hashCode());
        result = prime * result
                + ((validFrom == null) ? 0 : validFrom.hashCode());
        result = prime * result + ((vehicle == null) ? 0 : vehicle.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (!(obj instanceof TraInsurance))
            return false;
        TraInsurance other = (TraInsurance) obj;
        if (id == null) {
            if (other.id != null)
                return false;
        } else if (!id.equals(other.id))
            return false;
        if (validFrom == null) {
            if (other.validFrom != null)
                return false;
        } else if (!validFrom.equals(other.validFrom))
            return false;
        if (vehicle == null) {
            if (other.vehicle != null)
                return false;
        } else if (!vehicle.equals(other.vehicle))
            return false;
        return true;
    }  

}

Answer 1

请在此用例中明确使用JOIN ：

SELECT ve, MAX(v.validTill)
FROM TraInsurance v JOIN v.vehicle ve
GROUP BY ve
ORDER BY MAX(v.validTill)

Answer 2

显然 JPA 规范允许这样做，但至少 Hibernate 的实现不支持它（参见HHH-2436和HHH-1615 ）。

Answer 3

如果您在 GROUP BY 中传递一个实体，Hibernate 会自动将其id添加到底层数据库的转换后的 SQL 中。 此外，GROUP BY 中的值必须存在于 SELECT 子句中。 因此，您可以选择它的 id，而不是选择整个对象，然后从这些ids 中，您可以再次检索该对象。

 SELECT DISTINCT v.vehicle.id, max(v.validTill)
 FROM TraInsurance v 
 GROUP BY v.vehicle.id
 ORDER BY max(v.validTill)

如果需要时间并需要 DB 命中才能从其 id 中检索Vehicle对象，您可以在 SELECT 中选择Vehicle的所有属性并将它们放在 GROUP BY 中。 然后，您可以根据这些属性构造Vehicle对象，而无需访问 DB。