hibernate生成查询而不需要所有外部联接获取

Question

我有5个实体：

@Entity
public class A {
    @OneToMany(mappedBy="a", fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private Set<B1> b1s;

    @OneToMany(mappedBy="a", fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private Set<B2> b2s;
}

@Entity
public class B1 {
    @ManyToOne
    @JoinColumn(name="c")
    private C c;
}

@Entity
public class B2 {
    @ManyToOne
    @JoinColumn(name="c")
    private C c;
}

@Entity
public class C {
    @OneToMany(mappedBy="d", fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private Set<D> ds;
}

@Entity
public classD {

}

所以简而言之，一个急切地连接两个实体集B1和B2，每一个急切地连接（隐含地）一个实体C，而实体C又急切地连接一个实体集D。

加载A对象时，生成的查询如下所示

select (...) from A a0 
left outer join B1 b1 on a0.id=b1.aid
    left outer join C c1 on b1.cid=c1.id
        left outer join D d1 on c1.id=d1.cid
left outer join B2 b2 on a0.id=b2.aid
    left outer join C c2 on b2.cid=c2.id
where a0.id=?

Answer 1

的确，这似乎是一个bug或一个不完整的特性。我还能够在一个更简单的场景中重现它：

@Entity
public class A {
    @OneToMany(fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    @JoinColumn
    private Set<B> bs1;

    @OneToMany(fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    @JoinColumn
    private Set<B> bs2;
}

@Entity
public class B {
    @OneToMany(fetch=FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    @JoinColumn
    private Set<C> cs;
}

@Entity
public class C {

}

按id加载A时，将生成以下联接：

from
    A a0_ 
left outer join
    B bs1x1_ 
        on a0_.id=bs1x1_.bs1_id 
left outer join
    C cs2_ 
        on bs1x1_.id=cs2_.cs_id 
left outer join
    B bs2x3_ 
        on a0_.id=bs2x3_.bs2_id 

因此，如果您真的想在一个查询中获取所有内容，就必须使用HQL：

select a from A a 
left join fetch a.b1s b1 
left join fetch b1.c c1 
left join fetch c1.ds 
left join fetch a.b2s b2 
left join fetch b2.c c2 
left join fetch c2.ds 
where a.id = ?