Question

我在NEWSGROUP表中获得了多余的行。如何更改实体或查询以防止这种情况？

架构：

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>

假设与数据库建立单一连接以使解决方案暂时保持简单，而我see that it's a complex topic，假设有一个简单的方法假设“正确”条件。

文章实体：

    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;
    }
}

不是每次都创建一个新的新闻组实体，而是如何首先查询现有的新闻组？

NEWSGROUP中的冗余行：

mysql> 
mysql> USE usenet;SELECT * FROM ARTICLE;SELECT * FROM NEWSGROUP;
Database changed
+----+---------------+--------------+
| ID | MESSAGENUMBER | NEWSGROUP_ID |
+----+---------------+--------------+
|  1 |             4 |            1 |
|  2 |             5 |            2 |
|  3 |             6 |            3 |
|  4 |             7 |            4 |
|  5 |             8 |            5 |
|  6 |             9 |            6 |
|  7 |            10 |            7 |
|  8 |            11 |            8 |
|  9 |             4 |            9 |
| 10 |             4 |           10 |
| 11 |             5 |           11 |
| 12 |             6 |           12 |
| 13 |             7 |           13 |
| 14 |             8 |           14 |
| 15 |             9 |           15 |
| 16 |            10 |           16 |
| 17 |            11 |           17 |
| 18 |             4 |           18 |
| 19 |             5 |           19 |
| 20 |             6 |           20 |
| 21 |             7 |           21 |
| 22 |             8 |           22 |
| 23 |             9 |           23 |
| 24 |            10 |           24 |
| 25 |            11 |           25 |
+----+---------------+--------------+
25 rows in set (0.00 sec)

+----+-------------------------------+
| ID | NEWSGROUP                     |
+----+-------------------------------+
|  1 | gwene.com.androidcentral      |
|  2 | gwene.com.androidcentral      |
|  3 | gwene.com.androidcentral      |
|  4 | gwene.com.androidcentral      |
|  5 | gwene.com.androidcentral      |
|  6 | gwene.com.androidcentral      |
|  7 | gwene.com.androidcentral      |
|  8 | gwene.com.androidcentral      |
|  9 | gwene.com.blogspot.emacsworld |
| 10 | gwene.com.blogspot.googlecode |
| 11 | gwene.com.blogspot.googlecode |
| 12 | gwene.com.blogspot.googlecode |
| 13 | gwene.com.blogspot.googlecode |
| 14 | gwene.com.blogspot.googlecode |
| 15 | gwene.com.blogspot.googlecode |
| 16 | gwene.com.blogspot.googlecode |
| 17 | gwene.com.blogspot.googlecode |
| 18 | gwene.com.economist           |
| 19 | gwene.com.economist           |
| 20 | gwene.com.economist           |
| 21 | gwene.com.economist           |
| 22 | gwene.com.economist           |
| 23 | gwene.com.economist           |
| 24 | gwene.com.economist           |
| 25 | gwene.com.economist           |
+----+-------------------------------+
25 rows in set (0.00 sec)

mysql>

Answer 1

你的问题是你一直在停止新闻组...当然如果你保存文章，新的'reduntant'新闻组将被插入到数据库中，因为我没有身份证。

提示（希望）：

在Article Countstructor内（或文章类之外并在其中添加）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，所以我可以给你一个提示：

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NewsGroup为null创建一个新的One，则另外添加您找到的那个。

@OneToMany表关系中的冗余行数据或实体

1 个答案: