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POJPA OneToMany : List vs Set
 

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    <p>I have two entities: <code>UserAccount</code> and <code>Notification</code>. These have a relationship as shown below.</p> <pre><code> public class UserAccount { @Id @Column(name = "USER_NAME") private String emailId; @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL) @JoinTable(name = "USERS_NOTIFICATIONS", joinColumns = { @JoinColumn(name = "USER_NAME") }, inverseJoinColumns = { @JoinColumn(name = "NOTIFICATION_ID") }) private List&lt;Notification&gt; notifications; //setters, getter, equals and hashcode } </code></pre> <p>Both <code>equals()</code> and <code>hashcode()</code> are overridden (generated by the IDE with business key/primary key).</p> <p>Given a <code>UserAccount</code>, when I add the first <code>Notification</code>, it results in an INSERT statement. But on further addition for the same <code>UserAccount</code>, it first deletes and then inserts:</p> <pre><code>Hibernate: delete from USERS_NOTIFICATIONS where USER_NAME=? Hibernate: insert into USERS_NOTIFICATIONS (USER_NAME, NOTIFICATION_ID) values (?, ?) Hibernate: insert into USERS_NOTIFICATIONS (USER_NAME, NOTIFICATION_ID) values (?, ?) Hibernate: insert into USERS_NOTIFICATIONS (USER_NAME, NOTIFICATION_ID) values (?, ?) //as many inserts as the notifications the user has </code></pre> <p>The same happens with every <code>UserAccount</code>. If I replace the <code>List</code> with <code>Set</code>, a normal INSERT occurs. I found the reason after reading this <a href="http://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html/ch20.html#performance-collections-mostefficientupdate" rel="nofollow noreferrer">documentation</a> and a <a href="http://assarconsulting.blogspot.in/2009/08/why-hibernate-does-delete-all-then-re.html" rel="nofollow noreferrer">blog</a>. </p> <p><strong>Observations from docs</strong></p> <ul> <li>In a unidirectional <code>@OneToMany</code> association, <code>Set</code> is preferred.</li> </ul> <blockquote> <p>It should be clear that indexed <code>Collections</code> and <code>Sets</code> allow the most efficient operations in terms of adding, removing and updating elements.</p> </blockquote> <ul> <li>In a bidirectional <code>@OneToMany</code> relationship (<code>@ManyToOne</code> managing), <code>List</code> and <code>Bags</code> are efficient.</li> </ul> <blockquote> <p><code>Bags</code> and <code>Lists</code> are the most efficient inverse <code>Collections</code>.</p> </blockquote> <hr> <p>Having said that, which is more preferable:</p> <ol> <li><p>A <code>Set</code> over a <code>List</code> in a unidirectional <code>@OneToMany</code> mapping?</p></li> <li><p>Or, do I have to tweak my domain model by adding a bidirectional relationship to use a <code>List</code>, especially when there are duplicates?</p></li> </ol>
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    1. COOne would have thought that you use a List when you want ordering, and use a Set when you don't; choose your model based on what is needed to be modelled ... irrespective of any persistence solution. Any persistence solution ought to be able to handle either
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    2. CO@DataNucleus: One would have thought that. One would also have thought that Hibernate would handle adding to a unidirectional map efficiently. One sometimes finds that ones thoughts were misguided!
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    3. CO@DataNucleas: Even i thought it was that way, but after getting the problem i mentioned above and going through the documentation i found that its something more than that.
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Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

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