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POJPA fetching one to many
 

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    <p>I have the following entities: Event and Attribute. An event can have many attributes. </p> <p>The problem I am having is, I can't seem to find a query that returns the events with their attributes in a single SQL query. I could have millions of events that need to be iterated through and I don't want to lazy load them all for obvious performance reasons.</p> <p>I've tried using fetch in the query, but it returns an event for every attribute. i.e. if an event had 2 attributes it would return 2 events each with one attribute. </p> <p>What I want is one event with 2 attributes.</p> <pre><code>SELECT e FROM Event e LEFT JOIN FETCH e.attributes </code></pre> <p>If I add DISTINCT it works, but then it creates a distinct SQL query which is extremely slow over large data sets.</p> <pre><code>public class Event { @OneToMany(mappedBy = "event", cascade = CascadeType.ALL, fetch = FetchType.LAZY) public Set&lt;Attribute&gt; getAttributes(){} } public class Attribute { @ManyToOne(cascade=CascadeType.ALL) @JoinColumn(name = "event_id", nullable = false) public Event getEvent() { return event; } } </code></pre> <p><strong>Solution</strong></p> <p>I couldn't find a JPA solution. Instead, I unwrapped the EntityManager to the Hibernate session and used the criteria's result transformer as @milkplusvellocet suggested. This retrieves distinct root entities without creating a distinct SQL query.</p> <pre><code>Session session = em.unwrap(Session.class); Criteria criteria = session.createCriteria(Event.class); criteria.setFetchMode("attributes", FetchMode.JOIN); criteria.setResultTransformer(CriteriaSpecification.DISTINCT_ROOT_ENTITY); </code></pre> <p>One thing to note is, I tried using HQL from the unwrapped session. I added DISTINCT to the query and set the result transformer to DISTINCT_ROOT_ENTITY similar to the criteria solution above. Doing it this way <strong>still</strong> created a distinct SQL query. </p>
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A row detail

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.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

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