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PODisplay attributes of many to many relationship coded with @OneToMany and @ManyToOne
 

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  1. PODisplay attributes of many to many relationship coded with @OneToMany and @ManyToOne
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    <p>I'm a newbie in Spring. I think it's better to explain my problem with a little example. Let's say I have two main classes: <code>User</code> and <code>Group</code>. A <code>User</code> can be part of more <code>Group</code>s and a <code>Group</code>, obviously, can have more <code>User</code>s. So the relationship between them is many-to-many. What I would like to show, is something like this (using JSTL):</p> <pre><code>&lt;c:forEach items="${groups}" var="group"&gt; &lt;c:out value="${group.name}"/&gt; (&lt;c:out value="${fn:length(group.users)}" /&gt;):&lt;br /&gt; &lt;c:forEach items="${groups.users}" var="user"&gt; &lt;c:out value="${user.name}"/&gt;&lt;br /&gt; &lt;/c:forEach&gt;&lt;br /&gt; &lt;/c:forEach&gt; </code></pre> <p>Basically, the output should be something like:<br /></p> <blockquote> <p>Random (2):<br /> Joe<br /> Bloggs<br /><br /> Star wars (5):<br /> Luke<br /> Chewbacca<br /> Darth Vader<br /> Princess Leia<br /> Yoda<br /><br /> Nintendo (3):<br /> Super Mario<br /> Metroid<br /> Zelda<br /><br /></p> </blockquote> <p>I initially coded it with the classic <code>@ManyToMany</code> annotation, using an additional table <code>user_has_group</code> (created and managed by JPA) and it was working perfectly.</p> <p>I needed to modify the structure since I needed the <code>user_has_group</code> table to have the <code>joined_date</code> column. To achieve it, I read online that the best solution is to create another class (i.e. <code>UserHasGroup</code>) and add the one-to-many relationships to this class from <code>User</code> and <code>group</code>. Doing so, it's possible to add additional attributes to the <code>UserHasGroup</code> class (and therefore additional columns to the <code>user_has_group</code> table). Something like:</p> <p>User:</p> <pre><code>@Entity @Table(name = "user") public class User { @Id @GeneratedValue @Column private int id; @Column private String alias; @OneToMany(cascade = CascadeType.ALL, mappedBy = "user") private List&lt;UserHasGroup&gt; userHasGroup = new ArrayList&lt;UserHasGroup&gt;(); // Constructors/getters/setters } </code></pre> <p>Group:</p> <pre><code>@Entity @Table(name = "`group`") public class Group { @Id @GeneratedValue @Column private int id; @Column private String name; @OneToMany(cascade = CascadeType.ALL, mappedBy = "group") private List&lt;UserHasGroup&gt; userHasGroup = new ArrayList&lt;UserHasGroup&gt;(); // Constructors/getters/setters } </code></pre> <p>UserHasGroup:</p> <pre><code>@Entity @Table(name = "user_has_group") public class UserHasGroup { @Id @GeneratedValue @Column private int id; @ManyToOne @JoinColumn(name = "user_id") private User user; @ManyToOne @JoinColumn(name = "group_id") private Group group; @Column @Temporal(TemporalType.DATE) private Date joinedDate; // Constructors/getters/setters } </code></pre> <p>So far, so good. All the tests run successfully and the functionality is maintained.</p> <p>But I'm facing a problem with JSTL. In fact, with this new structure is obviously not possible to do <code>group.users</code> to iterate through the users.</p> <p>What is the best way to reach the same functionality as before but with this new structure?</p> <p>Thank you.</p>
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