ManyToMany映射关系表是空白的（Hibernate新手）

Question

iam working on an app where the association between entities is as follows . Here comment is the owner and iteration and user are the inverse tables . The requirement is - A user can have many comments similarly a comment can be given by multiple users . Also an iteration can have multiple comments and a single comment can belong to multiple iterations .

The code is as follows -

Comments entity -

@Table(name="RCOMMENTS")
@Entity
public class RComment{

@Id
@Column(name="COMMENTID")
@GeneratedValue(strategy=GenerationType.AUTO)
private long commentid;

@Column(name="DESCRIPTION")
private String description;

@Column(name="TYPE")
private String type;

@ManyToMany(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
@JoinTable(name = "COMMENTS_USERS", joinColumns = { @JoinColumn(name = "COMMENTID") }, 
    inverseJoinColumns = { @JoinColumn(name = "USERID") })
private Set <RUsers> users = new HashSet<RUsers>();

@ManyToMany(cascade=CascadeType.ALL,fetch=FetchType.EAGER)
@JoinTable(name = "COMMENTS_ITERATIONS", joinColumns = { @JoinColumn(name = "COMMENTID") }, 
    inverseJoinColumns = { @JoinColumn(name = "ITERATIONID") })
private Set <RIteration> iteration = new HashSet<RIteration>();

Users-

@Entity
@Table(name = "RUSER")
public class RUsers {
@Id
@Column(name = "USERID")
@GeneratedValue(strategy=GenerationType.AUTO)
private long userid;
@Column(name = "USERNAME")
private String username;
@Column(name = "PASSWORD")
private String password;

@ManyToMany(mappedBy="users")
private Set <RComment> comment ;

Iteration -

@Table(name = "RITERATION")
@Entity
public class RIteration {  
 @Id
 @Column(name = "iterationid")
 @GeneratedValue(strategy=GenerationType.AUTO)
private long iterationid;
private Date startdate;
private Date enddate;
private long iter;

@ManyToMany(mappedBy="iteration")
private Set <RComment> comment = new HashSet<RComment>();

Test code -

RUsers user = (RUsers) session.get(RUsers.class, this.userid);
Set<RComment> comments = user.getComment();

Now the issue that iam facing is the mapping tables are empty . Also user.getComment() yields empty set

Before i start my web app my db entity tables prepopulated with dummy values . But during run time when i debug the code user.getcomment returns empty set .

Could anyone please help me out what could be the issue .

Answer 1

If you have an bidirectional relationship between your Entities, then Hibernate will pay attention read only one of the two sides when it stores them in the database.

The side of the relation that is the important one, is the one that's ManyToMany annotation does NOT contain the mappedBy variable. And that is the side where you need to assign the value:

RUsers user = ...
RComment comment = ...

comment.users.add(user);  <--- that is the assignment hat hibernate will store!

user.comments.add(comment); <--- that assigment will NOT been stored by hibernate, but hibernate will load it from database