Java @OneToMany表关系中的冗余行数据或实体

新闻组表中有多余的行。如何更改实体或查询以防止出现这种情况

模式：

mysql> 
mysql> USE usenet;SHOW TABLES;DESCRIBE ARTICLE;DESCRIBE NEWSGROUP;
Database changed
+------------------+
| Tables_in_usenet |
+------------------+
| ARTICLE          |
| NEWSGROUP        |
+------------------+
2 rows in set (0.00 sec)

+---------------+------------+------+-----+---------+----------------+
| Field         | Type       | Null | Key | Default | Extra          |
+---------------+------------+------+-----+---------+----------------+
| ID            | bigint(20) | NO   | PRI | NULL    | auto_increment |
| MESSAGENUMBER | int(11)    | YES  |     | NULL    |                |
| NEWSGROUP_ID  | bigint(20) | YES  | MUL | NULL    |                |
+---------------+------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| ID        | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| NEWSGROUP | varchar(255) | YES  |     | NULL    |                |
+-----------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> 

假设只与数据库建立一个连接，以使解决方案暂时保持简单，而我这样做的时候，可能有一种简单的方法假设“正确”的条件

该条款实体：

    package net.bounceme.dur.usenet.model;

    import java.io.Serializable;
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Set;
    import javax.mail.Folder;
    import javax.mail.Message;
    import javax.persistence.*;

    @Entity
    public class Article implements Serializable {

        private static final long serialVersionUID = 1L;
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;
        @Column
        private int messageNumber;
        @ManyToOne(cascade = CascadeType.PERSIST)
        private Newsgroup newsgroup;

        public Article() {
        }

        public Article(Message message, Folder folder) {
            messageNumber = message.getMessageNumber();
            newsgroup = new Newsgroup(folder); //need to ensure uniqueness
        }

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        @Override
        public int hashCode() {
            int hash = 0;
            hash += (id != null ? id.hashCode() : 0);
            return hash;
        }

        @Override
        public boolean equals(Object object) {
            // TODO: Warning - this method won't work in the case the id fields are not set
            if (!(object instanceof Article)) {
                return false;
            }
            Article other = (Article) object;
            if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
                return false;
            }
            return true;
        }

        @Override
        public String toString() {
            return "\nmessageNumber\t" + messageNumber;
        }

        public int getMessageNumber() {
            return messageNumber;
        }

        public void setMessageNumber(int messageNumber) {
            this.messageNumber = messageNumber;
        }
    }



And the Newsgroup entity:


package net.bounceme.dur.usenet.model;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;
import javax.mail.Folder;
import javax.persistence.*;

@Entity
public class Newsgroup implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @Column //@Unique @UniqueConstraint interface..?
    private String newsgroup;
    @OneToMany(mappedBy = "newsgroup", cascade = CascadeType.PERSIST)
    private Set<Article> articles = new HashSet<>();

    public Newsgroup() {
    }

    public Newsgroup(Folder folder) {
        newsgroup = folder.getFullName();//if row already exists, then what?
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Newsgroup)) {
            return false;
        }
        Newsgroup other = (Newsgroup) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return newsgroup;
    }
}

---

你的问题是你一直在浏览新闻组。。。当然，如果你保存了这篇文章，一个新的“reduntant”新闻组将被插入数据库，因为我没有ID

提示（希望如此）：

在Articles Countstructor内部（或在Article类外部并将其添加）搜索要添加到文章中的新闻组

我的问题是。。。我不知道“文件夹”是什么，所以我只能给你一个提示：

Query query = entityManager.createNativeQuery("SELECT * FROM Newsgroup WHERE "dunno what you want to select here" = ?1",
            Newsgroup.class);
query.setParameter(1, "your parameter);

Newsgroup group = (Newsgroup) query.getSingleResult();

---

如果新闻组为

null

请创建一个新的新闻组，否则请添加您找到的新闻组。

我不确定您是否正确理解了您的问题。。你什么时候得到多余的行。。。保存一篇文章时？为什么/在哪里创建新的新闻组实体？@Jenson是的，每篇文章都会创建一个新的新闻组实体，而这只有在它是真正新的或唯一的新闻组时才会发生。然后，您必须在文章的构造函数中搜索要添加的新闻组->来自数据库（如果为空，则创建一个）@使用EntityManager类型查询进行Jenson搜索？这不会造成数据库连接问题吗？难道你不能用这种方法得到几十个潜在的递归连接吗？虽然，如果它适用于我的用例，那么它就足够了：）谢谢。我也在看，但我会试试看。该文件夹是javax.mail.folder，但您的答案是有意义的。

Query query = entityManager.createNativeQuery("SELECT * FROM Newsgroup WHERE "dunno what you want to select here" = ?1",
            Newsgroup.class);
query.setParameter(1, "your parameter);

Newsgroup group = (Newsgroup) query.getSingleResult();