Hibernate JPA，连接多个表

Question

 @Entity
 public class A {
      private String name;
      private Int idA;
      ...
 } 

 @Entity
 public class B {
      private String lastName;
      private Int idB;
      ...
 }

 @Entity
 public class C {
      private String name;
      private String lastName;
      ...
 }

我的本机查询将类似于：“select*from a inner join b inner join c on a.ida=b.idb and b.lastname=c.lastname and a.name=c.name”

在hibernate中，如果要连接两个表，可以使用@joincolum，如果要通过表B连接表A和表C，可以使用@joincable。

因此在HQL中查询将看起来像：“from A A inner join A.b as b inner join A.c”，其中

@Entity
public class A {
...
@OneToMany
@JoinColumn(name="idB", referencedColumnName="idA")
private List<B> b;
...

@ManyToMany
@JoinTable(name = "B",
     joinColumns = {@JoinColumn(name="idB", referencedColumnName="ioA")},
     inverseJoinColumns = {@JoinColumn(name="lastName",referencedColumnName="lastName")}
 )
private List<C> c;
...
}

但在这种情况下，我没有从a表直接访问C表的权限，有人能解释一下我是怎么得到的吗

 select * from a inner join b inner join c on a.idA = b.idB and b.lastName = c.lastName and a.name = c.name 

在HQL和JPA中

Answer 1

要让JPA理解DB结构，还需要定义关系。

@Entity
 public class A {
      private String name;
      private Int idA;

      @OneToMany(fetch = FetchType.LAZY, mappedBy = "a")
      private List<B> bs = new ArrayList()<>;
      ...
 } 

 @Entity
 public class B {
      private String lastName;
      private Int idB;

      @ManyToOne
      private A a;

      @OneToMany(fetch = FetchType.LAZY, mappedBy = "b")
      private List<C> cs = new ArrayList()<>;
      ...
 }

 @Entity
 public class C {
      private String name;
      private String lastName;

      @ManyToOne
      private B b;
      ...
 }

然后你就可以做一些简单的事情

A a = yourARepository.findById(1);
List<B> bs = a.getBs();
For(B b : bs){
    List<C> cs = b.getCs();
}

那只是我头顶上的一个抄写员给你一个主意。您可能需要做一些调整。我可以试试，等我回家后再修。：）