Java jpa-如何创建与IdClass的多对多关系?
我有3张具有多对多关系的表Java jpa-如何创建与IdClass的多对多关系?,java,hibernate,jpa,many-to-many,Java,Hibernate,Jpa,Many To Many,我有3张具有多对多关系的表 (id、名称等)主键id B(id,名称,…)和主键id AB(id_a,id_b,日期,…)-带有否的关系表 主键 我是否可以为hibernate建模类,以便使用IdClass而不是EmbeddedId解决方案(从而避免使用级联不必要的级别来调用-getId().getA()) PS尝试混合这些解决方案,但无效: 在建立实体之间的关系时,我旁边最近的墙和我的头经常连接在一起。。。(斯蒂芬) 以下是实体a和B之间多对多关系的工作示例: A.java impor
- (id、名称等)主键id
- B(id,名称,…)和主键id
- AB(id_a,id_b,日期,…)-带有否的关系表 主键
在建立实体之间的关系时,
我旁边最近的墙和我的头经常连接在一起。。。(斯蒂芬) 以下是实体a和B之间多对多关系的工作示例: A.java
import java.util.ArrayList;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
@Entity
public class A {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
@Column(name="id_a")
private Integer id;
private String name;
@OneToMany(mappedBy="a",cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private List<AB> abAssociations = new ArrayList<>();
// Getters and setters...
}
import java.util.ArrayList;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
@Entity
public class B {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
@Column(name = "id_b")
private Integer id;
private String name;
@OneToMany(mappedBy = "b", cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private List<AB> abAssociations = new ArrayList<>();
// Getters and setters...
}
import java.util.Date;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.ManyToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entity
@IdClass(ABid.class)
public class AB {
@Id
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private A a;
@Id
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private B b;
@Temporal(TemporalType.TIMESTAMP)
private Date date;
// Getters and setters...
}
import java.io.Serializable;
// The IdClass MUST implement Serializable and override #hashCode and #equals
public class ABid implements Serializable {
private static final long serialVersionUID = -2834827403836993112L;
private A a;
private B b;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((a == null) ? 0 : a.hashCode());
result = prime * result + ((b == null) ? 0 : b.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
ABid other = (ABid) obj;
if (a == null) {
if (other.a != null)
return false;
} else if (!a.equals(other.a))
return false;
if (b == null) {
if (other.b != null)
return false;
} else if (!b.equals(other.b))
return false;
return true;
}
}
ABid.java
import java.util.ArrayList;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
@Entity
public class A {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
@Column(name="id_a")
private Integer id;
private String name;
@OneToMany(mappedBy="a",cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private List<AB> abAssociations = new ArrayList<>();
// Getters and setters...
}
import java.util.ArrayList;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
@Entity
public class B {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
@Column(name = "id_b")
private Integer id;
private String name;
@OneToMany(mappedBy = "b", cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private List<AB> abAssociations = new ArrayList<>();
// Getters and setters...
}
import java.util.Date;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.ManyToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entity
@IdClass(ABid.class)
public class AB {
@Id
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private A a;
@Id
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private B b;
@Temporal(TemporalType.TIMESTAMP)
private Date date;
// Getters and setters...
}
import java.io.Serializable;
// The IdClass MUST implement Serializable and override #hashCode and #equals
public class ABid implements Serializable {
private static final long serialVersionUID = -2834827403836993112L;
private A a;
private B b;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((a == null) ? 0 : a.hashCode());
result = prime * result + ((b == null) ? 0 : b.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
ABid other = (ABid) obj;
if (a == null) {
if (other.a != null)
return false;
} else if (!a.equals(other.a))
return false;
if (b == null) {
if (other.b != null)
return false;
} else if (!b.equals(other.b))
return false;
return true;
}
}
pom.xml
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-entitymanager</artifactId>
<version>4.3.7.Final</version>
</dependency>
下面是Hibernate在Postgresql数据库上创建的表的sql代码
CREATE TABLE a
(
id_a integer NOT NULL,
name character varying(255),
CONSTRAINT a_pkey PRIMARY KEY (id_a)
)
CREATE TABLE b
(
id_b integer NOT NULL,
name character varying(255),
CONSTRAINT b_pkey PRIMARY KEY (id_b)
)
CREATE TABLE ab
(
date timestamp without time zone,
b_id_b integer NOT NULL,
a_id_a integer NOT NULL,
CONSTRAINT ab_pkey PRIMARY KEY (a_id_a, b_id_b),
CONSTRAINT fk_3exna7nsxvj1kv9i9pntmwlf1 FOREIGN KEY (a_id_a)
REFERENCES a (id_a) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION,
CONSTRAINT fk_n3jrq53nr1elew4rytocopkbu FOREIGN KEY (b_id_b)
REFERENCES b (id_b) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION
)
回应记者的评论
试试这个:
@Entity
public class AB {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
@Column(name = "id_ab")
private Integer id;
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private A a;
@ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
private B b;
@Temporal(TemporalType.TIMESTAMP)
private Date date;
// Getters and setters...
}
太棒了,谢谢你!(这似乎是一个小问题,如果您不在AB类中为两个字段添加@JoinColumn(name=“a”,nullable=false,updateable=false)注释,a和b)您保存了我的日期:)如果我希望AB类有自己的ID,我该怎么办incremental@valik请看我的回答: