Hibernate级联多对多在子引用中创建副本

问题描述

好的，所以我对hibernate很陌生。
问题是关于级联多对多，避免添加重复值。
所以我遵循这个例子。

问题是，如果我运行程序两次，它会将重复值添加到证书表。

值给员工表。它级联并向证书表插入值：

  id证书名称
 1 PMP 
 2 MBA 
 3 MCA 
  

在我运行这个例子后，行动。

  id证书名称
 1 PMP 
 2 MBA 
 3 MCA 
 4 PMP 
 5 MBA 
 6 MCA 
  

但是证书表具有共享值。值为4-6和1-3一样。

我试图在表格证书上添加唯一约束，但是我得到这个错误：

我如何插入值不复制它们。
这可以避免，还是我必须使用其他技术来做到这一点。

如果需要使用jar，请添加pom.xml。

  < project xmlns =http://maven.apache.org/POM/4.0.0xmlns：xsi =http://www.w3.org/2001/XMLSchema-instancexsi：schemaLocation =http： //maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd\"> 
< modelVersion> 4.0.0< / modelVersion> 
< groupId> testHibernateCascade2< / groupId> 
< artifactId> testHibernateCascade2< / artifactId> 
< version> 0.0.1-SNAPSHOT< / version> 
< build> 
< sourceDirectory> src< / sourceDirectory> 
< plugins> 
< plugin> 
< artifactId> maven-compiler-plugin< / artifactId> 
< version> 3.1< / version> 
<配置> 
< source> 1.6< / source> 
< target> 1.6< / target> 
< / configuration> 
< / plugin> 
< / plugins> 
< / build> 
 
<依赖关系> 
< dependency> 
< groupId> org.hibernate< / groupId> 
< artifactId> hibernate-core< / artifactId> 
< version> 4.3.6.Final< / version> 
< /依赖关系> 
 
< dependency> 
< groupId> mysql< / groupId> 
< artifactId> mysql-connector-java< / artifactId> 
< version> 5.1.31< / version> 
< /依赖关系> 
< /依赖关系> 
 
< / project> 
  

解决方案

好的，我找到了解决方案。
首先，我使用HQL和Criteria检查数据库中是否存在这样的证书。
$ b

  public int getID（String certificateName） {
 
 Criteria cr = session.createCriteria（Certificate.class）; 
 cr.add（Restrictions.eq（certificate，certificateName））; 
列表<？> results = cr.list（）; 
 if（results.isEmpty（））{
 return -1;} 
 else {
 Iterator<？> iterator = results.iterator（）; 
证书证书=（证书）iterator.next（）; 
返回certificate.getId（）; 
 
 
 
 
 
 
其次如果certificate_name不在 certificate 表我创建新对象，否则我添加现有的。
  ... 
 String [] userCertificates = { BKA， RRA， DMA}; 
 
 HashSet< Certificate> certificatesSet = new HashSet< Certificate>（）; 
 
 for（int i = 0; i< userCertificates.length; i ++）{
 int id = getID（userCertificates [i]）; 
 if（id == -1）{
 certificatesSet.add（new Certificate（userCertificates [i]））; 
} 
 else {
 certificatesSet.add（（Certificate）cs.getObject（Certificate.class，id））; 
} 
} 
 ... 
  
好。
 
Ok so I'm new to hibernate.The question is about cascade many-to-many, avoiding to add duplicate values.So I follow this example.tutorialspoint hibernate many to many mapping
The problem is this, that if I run program twice it adds duplicate values to certificate table.
After I insert values to employee table. It cascades and inserts values to certificate table:
id certificate_name
1   PMP
2   MBA
3   MCA
After I run this example second time it does the same actions.
id certificate_name
1   PMP
2   MBA
3   MCA
4   PMP
5   MBA
6   MCA
But then certificate table has dublicate values. Values 4-6 are the same as 1-3.
I tried to add unique constraint on table certificate, but then I get this error:
How can I insert values not to duplicate them.Can this be avoided, or do I have to use other techniques to do that. 
Also add pom.xml if needed to use jars.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>testHibernateCascade2</groupId>
  <artifactId>testHibernateCascade2</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <build>
    <sourceDirectory>src</sourceDirectory>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.1</version>
        <configuration>
          <source>1.6</source>
          <target>1.6</target>
        </configuration>
      </plugin>
    </plugins>
  </build>

  <dependencies>
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>4.3.6.Final</version>
    </dependency>

    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.31</version>
    </dependency>
  </dependencies>  

</project>
 解决方案 
Ok, I found the solution.First I check if such certificate already exists in DB using HQL and Criteria.
public int getID(String certificateName){

    Criteria cr = session.createCriteria(Certificate.class);
    cr.add(Restrictions.eq("certificate", certificateName));
    List<?> results = cr.list();
    if (results.isEmpty()){ 
        return -1;}
        else{
            Iterator<?> iterator = results.iterator();
            Certificate certificate = (Certificate) iterator.next();
            return certificate.getId();
        }
}
Second if certificate_name is not in certificate table I create new object, else I add existing.
    ...
    String [] userCertificates = {"BKA","RRA","DMA"};

    HashSet<Certificate> certificatesSet = new HashSet<Certificate>();

    for (int i = 0; i<userCertificates.length;i++){
        int id = getID(userCertificates[i]);
        if (id == -1){
            certificatesSet.add(new Certificate(userCertificates[i]));
        }
        else{
            certificatesSet.add((Certificate) cs.getObject(Certificate.class, id));
        }
    }
    ...
And that worked very well.
                        
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