如何使用Hibernate / JPA从另一个表中从另一个表中检索一列

Question

我想使用外键“ model_id”仅检索一个列“ model_name”从tt_cars表中， 我尝试了以下代码，但它返回了整个汽车对象。

@JoinColumn(name = "MODEL_ID", referencedColumnName = "ID")
@ManyToOne(fetch = FetchType.EAGER)
private CARS cars;

另外，我尝试了下面的代码，它也无法正常工作

@SecondaryTable(name = "TT_CARS", pkJoinColumns = @PrimaryKeyJoinColumn(name = "ID", referencedColumnName="MODEL_ID"))

，还有其他方法可以使用Hibernate和JPA保留列（model_name）？

备注：模型名称应为选项类的一部分。

我的代码

import javax.persistence.*;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

@Entity
@Table(name = "TT_OPTIONS")
public class Options  {

    @Id
    @Column(name = "ID")
    private String id;

    @NotNull
    @Column(name = "DESCRIPTION", nullable = false)
    private String description;

    @Column(name = "MODEL_ID") // Foreign key
    private Long modelId;

    @Column(name = "MODEL_NAME", table = "TT_CARS") // this is the column name I would like to retrieve from the TT_CARS table
    private String modelName;

    // getters and setters
}

Answer 1

您可以使用@noreflow noreferrer“>@formula 。它是仅读取的列，可以通过自定义子查询检索。它不存在于目标表中。

定义一个公式（派生值），该公式是一个SQL片段，充当
在大多数情况下，@column替代方案。代表只读状态。

示例：

@Entity
@Table(name = "TT_OPTIONS")
public class Options  {

    @Id
    @Column(name = "ID")
    private Long id;

    @Column(name = "DESCRIPTION", nullable = false)
    private String description;

    @Column(name = "MODEL_ID") 
    private Long modelId;

    @Formula("(select TT_CARS.MODEL_NAME from TT_CARS where TT_CARS.ID = MODEL_ID)")
    private String modelNameFormula;
}

@Entity
@Table(name = "TT_CARS")
public class Cars {
    @Id
    @Column(name = "ID")
    private Long id;

    @Column(name = "MODEL_NAME")
    private String modelName;
}

Hibernate生成的本机查询：

    select
        options0_.id as id1_4_0_,
        options0_.description as descript2_4_0_,
        options0_.model_id as model_id3_4_0_,
        (select
            TT_CARS.MODEL_NAME 
        from
            TT_CARS 
        where
            TT_CARS.ID = options0_.MODEL_ID) as formula1_0_ 
    from
        tt_options options0_ 
    where
        options0_.id=?