java - LEFT JOIN FETCH 不起作用

Question

如何在 JPA/Eclipselink 2.6.5 中使用三级“LEFT JOIN”获取 Table3(关系 OneToOne)？

此命令无法获取它:(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

这可以获取它，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!

Answer 1

JPA/JPQL 不支持嵌套获取联接。要解决此问题，您可以添加 query hint到您的查询，告诉它您想要使用左外连接连接什么。 fetch-join查询提示有一个更好的示例来展示它的使用方式。

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");