EclipseLink:如何避免对嵌套左连接的附加SQL查询 [英] EclipseLink:How to avoid additional sql query for nested left join

问题描述

我有以下实体:

@Entity
class A{
 @OneToMany
 private List<B> bs;
 ...
}

@Entity
class B{
 @OneToMany
 private List<C> cs;
 ...
}

@Entity
class C{
 ...
}

所以我执行以下查询:

SELECT a FROM A a LEFT JOIN FETCH a.bs b LEFT JOIN b.cs

此代码有效，唯一的问题是在一个联接查询中从数据库读取A和B，但要读取C(LEFT JOIN b.cs)，则执行单独的sql查询以仅读取C实体.如何在一个sql查询中读取A，B，C.

This code works, the only problem that A and B are read from database in one join query, but for reading C (LEFT JOIN b.cs) separate sql query is executed to read only C entities. How to read A,B,C in one sql query.

推荐答案

JPA不允许嵌套的访存联接，但是您可以使用特定于EclipseLink的此答案

JPA does not allow nested fetch joins but you can use the EclipseLink specific left-join-fetch query hint to tell it you want the b.cs relationship fetched. See this answer

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