Java hibernate和mappedBy:是否可以在不设置对象之间双向关系的情况下自动设置外键?
欢迎, 我有两门课:对话和提问。一次谈话有很多问题 Conversation.java:Java hibernate和mappedBy:是否可以在不设置对象之间双向关系的情况下自动设置外键?,java,hibernate,hibernate-mapping,mappedby,Java,Hibernate,Hibernate Mapping,Mappedby,欢迎, 我有两门课:对话和提问。一次谈话有很多问题 Conversation.java: package com.jcg.jpa.mappedBy; import java.io.Serializable; import java.util.ArrayList; import java.util.Collection; import javax.persistence.CascadeType; import javax.persistence.Column; import javax.per
package com.jcg.jpa.mappedBy;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Collection;
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;
import javax.persistence.Table;
@Entity
@Table(name = "CONVERSATION_TABLE")
public class Conversation implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "CONV_ID")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int conversationId;
@Column(name = "CONV_NAME")
private String name;
@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy="conversation")
private Collection<Question> questions = new ArrayList<Question>();
public Conversation() { }
public int getConversationId() {
return conversationId;
}
public void setConversationId(int conversationId) {
this.conversationId = conversationId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Collection<Question> getQuestions() {
return questions;
}
public void setQuestions(Collection<Question> questions) {
this.questions = questions;
}
@Override
public String toString() {
return "Employee [conversationId=" + conversationId + ", name=" + name + "]";
}
}
package com.jcg.jpa.mappedBy;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
@Entity
@Table(name = "QUESTION_TABLE")
public class Question implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
@ManyToOne
@JoinColumn(name = "CONVERSATION_CONV_ID", nullable = false)
private Conversation conversation;
@Column(name = "QUESTION_TEXT")
private String questionText;
@Column(name = "ANSWER_TEXT")
private String answerText;
public Question() { }
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getQuestionText() {
return questionText;
}
public void setQuestionText(String questionText) {
this.questionText = questionText;
}
public String getAnswerText() {
return answerText;
}
public void setAnswerText(String answerText) {
this.answerText = answerText;
}
public Conversation getConversation() {
return conversation;
}
public void setConversation(Conversation conversation) {
this.conversation = conversation;
}
@Override
public String toString() {
return "Question [id=" + id + ", questionText=" + questionText
+ ", answerText=" + answerText +"]";
}
}
persistence.xml:
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="JPAMappedbyExample" transaction-type="RESOURCE_LOCAL">
<class>com.jcg.jpa.mappedBy.Conversation</class>
<class>com.jcg.jpa.mappedBy.Question</class>
<!-- Configuring The Database Connection Details -->
<properties>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpatest" />
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password" value="qwerty" />
</properties>
</persistence-unit>
接下来,创建一个包含这两个问题的问题列表
并设置为conv问题列表:
conv.setQuestions(questions);
@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy="conversation")
private Collection<Question> questions = new ArrayList<Question>();
一个it工作正常,因为数据被插入到两个表中,
并且外键对话会话ID已填充:
但是,当我删除问题中的设置对话时,行:
question1.setConversation(conv);
question2.setConversation(conv);
外键设置为NULL。为什么?我们已经在对话问题列表中添加了两个问题:
conv.setQuestions(questions);
@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy="conversation")
private Collection<Question> questions = new ArrayList<Question>();
@OneToMany(fetch=FetchType.EAGER,cascade=CascadeType.ALL,mappedBy=“conversation”)
私人收藏问题=新建ArrayList();
,所以hibernate应该知道这两个问题的对话外键是什么(因为它们位于指定的对话问题列表中)。那么,是否可以避免为每个问题设置对话,而只将问题添加到对话的问题列表中?我应该如何配置实体来实现这一点?或者可能这是不可能的,我们总是需要设定两个方向
所以hibernate应该知道这两个问题的对话外键是什么(因为它们位于指定的对话问题列表中)
如果不使用setConversation()
指定,任何Hibernate都不应该知道这两个问题的对话
,因为它在这里处理对象,而这两个问题
没有任何指示它们属于哪个对话
说明:
因为当您仅将这两个问题
添加到对话
对象时,此信息在问题
对象中不可见
这里的另一件事是,mappedBy
属性指示object
是映射的所有者,因此如果mappedBy
侧没有给定的对象,如何进行映射
这就是为什么您应该在问题
对象中指定对话
,以便可以通过休眠
正确评估映射
注意:
建议对OneToMany
映射使用集
,而不是任何其他集合
,因为此集合不应该有任何重复项,因此您最好将其更改为:
@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy="conversation")
private Set<Question> questions = new HashSet<Question>();
@OneToMany(fetch=FetchType.EAGER,cascade=CascadeType.ALL,mappedBy=“conversation”)
私有集问题=新HashSet();