避免Hibernate将一对多（或一对多）关联映射到数据库表中的关系表

问题描述

我是Hibernate的新手。我注意到在Hibernate中，将java类映射到数据库表通常涉及关系表，即使有时候关系表也不是必需的（就像在一对多关系中或相反）。

例如：

我是一个公司类和一个Flight类，其中一个公司可以有很多航班（从公司到航班的一对多关联）。

我有以下使用hibernate注释的代码：

  @Entity 
 @Table（name =COMPANY）
 public class Company {
 
 @Id 
 private Long ID; 
 
 @OneToMany 
私人设置< Flight>航班=新的HashSet< Flight>（）; 
 
 ...... 
获取和设置方法
 ...... 
} 
 
 
 
 @Entity 
 @Table（name =FLIGHT）
 public class Flight {
 @Id 
 private Long ID; 
 
 @ManyToOne 
 @JoinColumn（name =COMP_ID）
 private公司ownerCompany; 
 
 ...... 
 getter和setter方法
 ...... 
 
} 
  

这些类已成功映射到数据库中。有三个表格，分别是：

1. 公司（ID字段） ID字段和COMP_ID字段）


2. COMPANY_MANY_TO_ONE_FLIGHT（两个字段：MANY_TO_ONE_COMPANY_id和flights_id）

然而，最后一个表COMPANY_MANY_TO_ONE_FLIGHT是一个由hibernate添加的关系表，它是多余的。

显然，在FLIGHT表中有一个外键COMP_ID，这是合理的删除冗余关系表。

我怎样才能避免这种情况？

解决方案

尝试在@OneToMany注释中使用mappedBy属性：

  @OneToMany（mappedBy =ownerCompany）
私人设置< Flight>航班=新的HashSet< Flight>（）; 
  

您可以在这里查看常见关联mappend和hibernate注释：

I am new to Hibernate. I notice that in Hibernate, mapping the java classes into database tables often involve relation tables, even sometimes relation tables are not necessary(Like in a one-to-many relation or the opposite).

For example:

I am a Company class and a Flight class, in which a company can have many flights(a one to many association from Company to Flight).

I have the following code using hibernate annotations:

@Entity
@Table(name = "COMPANY")
public class Company {

    @Id
    private Long id;

    @OneToMany
    private Set<Flight> flights = new HashSet<Flight>();

    ......
    getter and setter methods
    ......
}



@Entity
@Table(name="FLIGHT")
public class Flight{
    @Id
    private Long id;

    @ManyToOne
    @JoinColumn(name = "COMP_ID")
    private Company ownerCompany;

    ......
    getter and setter methods
    ......

}

The classes are successfully mapped into database. And there are three tables, which are:

1. COMPANY(an ID field)
2. FLIGHT(an ID field and an COMP_ID field)
3. COMPANY_MANY_TO_ONE_FLIGHT(two fields:MANY_TO_ONE_COMPANY_id and flights_id )

However, the last table COMPANY_MANY_TO_ONE_FLIGHT is a relation table added by hibernate, which is redundant.

Obviously, there is a foreign key COMP_ID in FLIGHT table and it is reasonable remove the redundant relation table.

And how can I avoid such circumstance? Like through modifying the annotations.

解决方案

try using the mappedBy property in the @OneToMany annotation:

@OneToMany(mappedBy="ownerCompany")
private Set<Flight> flights = new HashSet<Flight>();

you can look up common associations mappend with hibernate annotations here:http://docs.jboss.org/hibernate/stable/annotations/reference/en/html/entity.html#entity-mapping-association

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