我有一个关于mysql java pojo的问题。我试图在一列中插入多个值。但是代码抛出了空值。这些sql查询不是在jpa上使用的，而是在r2dbc spring webflux上使用的。正如你所知道的，r2dbc驱动程序不支持表关系，所以我试图直接插入值到每个表中。首先我做了pojo代码。

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
@Table
public class Post {
 
    @Id
    @Column("post_id")
    private Long postId;
    
    @Column
    private String title;
  
    @Column
    private String body;
    
    @Column("tag_id")
    private Collection<Long> tagId;  // value of tagId is null
}

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
@Table
public class Tag {
  
    @Id
    @Column("tag_id")
    private Long tagId;

    @Column
    private String body;
    
    @Column("post_id")
    private Long postId;
}

然后执行下面的SQL语句

CREATE TABLE IF NOT EXISTS post (
    post_id BIGINT NOT NULL AUTO_INCREMENT,
    title VARCHAR(30) NOT NULL,
    body TEXT,
    
    PRIMARY KEY (post_id)
);

CREATE TABLE IF NOT EXISTS tag (
    tag_id BIGINT NOT NULL AUTO_INCREMENT,
    body VARCHAR(255),
    post_id BIGINT NOT NULL,

    PRIMARY KEY (tag_id),
    CONSTRAINT tag_ibfk_1 FOREIGN KEY (post_id) REFERENCES post (post_id) ON DELETE CASCADE
);

post和tag表的表关系是一对多，下面的代码是sql插入代码。

INSERT INTO post (post_id, title, body) VALUES (1, 2, 'Title 1', 'post #1 body');
INSERT INTO post (post_id, title, body) VALUES (2, 2, 'Title 2', 'post #2 body');

INSERT INTO tag (tag_id, post_id, body) VALUES (1, 1, 'first tag');
INSERT INTO tag (tag_id, post_id, body) VALUES (2, 1, 'second tag');
INSERT INTO tag (tag_id, post_id, body) VALUES (3, 2, 'third tag');
INSERT INTO tag (tag_id, post_id, body) VALUES (4, 2, 'last tag');

以上SQL语句在项目启动时执行，'post' java类的'tagId'成员变量为java Collection类型，即有多个标签值，但当React网响应返回时，标签值为空。

http://localhost:8080/route/post/all

[{"postId":1,"title":"Title 1","body":"post #1 body","tagId":null},{"postId":2,"title":"Title 2","body":"post #2 body","tagId":null}]

如何将标签类的多个tag_id值插入post.tagId？有什么想法吗？