Mybatis如何实现一对多或多对一的ORM映射呢？

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1.我们准备两个实体类，一个是Blog类，另一个是Comment类，通过对比实体关系，我们知道，一篇博客可以有多条评论，所以是一对多的关系，所以评论对于博客是多对一的关系。

我们可以通过外键约束来简历实体之间的联系，如简历评论中的一个blog_id字段对应博客的主键字段。

2.Blog.java,通过JPA的注解来实现数据库表的映射。

@Entity
@Table(name = "t_blog")
public class Blog {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    private String title;

    @Basic(fetch = FetchType.LAZY) //可以没有
    @Lob
    private String content;
    private String firstPicture;
    private String flag;
    private Integer views;
    private boolean appreciation;
    private boolean shareStatement;
    private boolean commentabled;
    private boolean published;
    private boolean recommend;
    @Temporal(TemporalType.TIMESTAMP)
    private Date createTime;
    @Temporal(TemporalType.TIMESTAMP)
    private Date updateTime;

    @ManyToOne
    private Type type;

    @ManyToMany(cascade = {CascadeType.PERSIST})
    private List<Tag> tags = new ArrayList<>();


    @ManyToOne
    private User user;

    @OneToMany(mappedBy = "blog")
    private List<Comment> comments = new ArrayList<>();

    @Transient
    private String tagIds;

    private String description;
    
    //省略 对外方法 getter setter 构造 toString
}

3.Comment.java,评论的实体类

@Entity
@Table(name = "t_comment")
public class Comment {

    @Id
    @GeneratedValue
    private Long id;
    private String nickname;
    private String email;
    private String content;
    private String avatar;
    @Temporal(TemporalType.TIMESTAMP)
    private Date createTime;

    @ManyToOne
    private Blog blog;

    @OneToMany(mappedBy = "parentComment")
    private List<Comment> replyComments = new ArrayList<>();

    @ManyToOne
    private Comment parentComment;

    private boolean adminComment;
    
    //省略对外方法
}

4.Mybatis的mapper接口

/**
 * @author 王一宁
 * @date 2020/3/11 15:30
 */
@Mapper
public interface CommentMapper {
    /*查询所有的评论信息*/
    //@Select("SELECT a.content,a.email,a.nickname,a.create_time,b.title from t_comment a,t_blog b WHERE a.blog_id=b.id ORDER BY a.create_time DESC LIMIT 30;")
    List<Comment> findAll();
}

5.CommentMapper.xml 为实现sql的文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wang.springboot.mapper.CommentMapper">

    <!-- 一对多 级联查询的第一种方法（嵌套查询） -->
    <resultMap type="com.wang.springboot.pojo.Comment" id="blogAndComments1">
        <id property="content" column="content" />
        <result property="email" column="email" />
        <result property="nickname" column="nickname" />
        <result property="createTime" column="create_time" />
        <collection property="blog" ofType="com.wang.springboot.pojo.Blog">
            <result column="title" property="title"/>
        </collection>
    </resultMap>
    <!-- 这里我们还可以通过左外连接的方式进行查询，这里不做展示，直接通过关联查询 -->
    <select id="findAll"  resultMap="blogAndComments1">
        SELECT a.content,a.email,a.nickname,a.create_time,b.title from t_comment a,t_blog b WHERE a.blog_id=b.id ORDER BY a.create_time DESC LIMIT 30
    </select>

</mapper>