左联接取不起作用

如何在JPA/lippselink 2.6.5中使用三个级别的"LEFT JOIN"来获取表3(Relationship ONEtoONE)？

此命令不获取它：(错误)

SELECT t1 FROM Table1 t1 LEFT JOIN FETCH t1.table2 t2
                         LEFT JOIN FETCH t2.table3

此命令获取它，但不保留第一个"Left"：(错误)

SELECT t1 FROM Table1 t1 LEFT JOIN FETCH t1.table2
                         LEFT JOIN FETCH t1.table2.table3

这将获取它，但它只有两个级别：(不需要)

SELECT t2 FROM Table2 t2 LEFT JOIN FETCH t2.table3

表1：

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "Id")
private Integer id;

@JoinColumn(name = "Table2", referencedColumnName = "Id")
@ManyToOne(fetch = FetchType.LAZY)
private Table2 table2;

表2：

@Id
@Basic(optional = false)
@NotNull
@Column(name = "Id")
private Integer id;

@OneToMany(cascade = CascadeType.ALL, mappedBy = "table2")
private Collection<Table1> table1Collection;

@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "table2")
private Table3 table3;

表3：

@Id
@Basic(optional = false)
@NotNull
@Column(name = "Id")
private Integer id;

@JoinColumn(name = "Table2", referencedColumnName = "Id", insertable = false, updatable = false)
@OneToOne(fetch = FetchType.LAZY, optional = false)
private Table2 table2;

谢谢！

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解决方案

JPA/JPQL不支持嵌套的FETCH联接。要解决这个问题，您可以在查询中添加一个query hint，以告诉它您希望使用左外部联接来联接什么。fetch-join查询提示有一个更好的示例，说明如何使用它。

Query query = em.createQuery("SELECT t1 FROM Table1 t1", Table1.class);
query.setHint("eclipselink.LEFT_FETCH", "t1.table2");
query.setHint("eclipselink.LEFT_FETCH", "t1.table2.table3");