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>
