11.11. FetchMode.JOIN

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To demonstrate how `FetchMode.JOIN` works, we are going to modify the [`FetchMode.SELECT` mapping example](#fetching-strategies-fetch-mode-select-mapping-example) to use
`FetchMode.JOIN` instead:

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<div id="fetching-strategies-fetch-mode-join-mapping-example" class="exampleblock">
<div class="title">Example 296. `FetchMode.JOIN` mapping example</div>
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<pre class="prettyprint highlight">`@OneToMany(mappedBy = "department")
@Fetch(FetchMode.JOIN)
private List&lt;Employee&gt; employees = new ArrayList&lt;&gt;();`</pre>
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Now, we are going to fetch one `Department` and navigate its `employees` collections.

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The reason why we are not using a JPQL query to fetch multiple `Department` entities is because
the `FetchMode.JOIN` strategy would be overridden by the query fetching directive.

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To fetch multiple relationships with a JPQL query, the `JOIN FETCH` directive must be used instead.

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Therefore, `FetchMode.JOIN` is useful for when entities are fetched directly, via their identifier or natural-id.

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Also, the `FetchMode.JOIN` acts as a `FetchType.EAGER` strategy.
Even if we mark the association as `FetchType.LAZY`, the `FetchMode.JOIN` will load the association eagerly.

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Hibernate is going to avoid the secondary query by issuing an OUTER JOIN for the `employees` collection.

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<div id="fetching-strategies-fetch-mode-join-example" class="exampleblock">
<div class="title">Example 297. `FetchMode.JOIN` mapping example</div>
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<div class="listingblock">
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<pre class="prettyprint highlight">`Department department = entityManager.find( Department.class, 1L );

log.infof( "Fetched department: %s", department.getId());

assertEquals( 3, department.getEmployees().size() );`</pre>
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<pre class="prettyprint highlight">`SELECT
    d.id as id1_0_0_,
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_2_,
    e.department_id as departme3_1_2_,
    e.username as username2_1_2_
FROM
    Department d
LEFT OUTER JOIN
    Employee e
        on d.id = e.department_id
WHERE
    d.id = 1

-- Fetched department: 1`</pre>
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This time, there was no secondary query because the child collection was loaded along with the parent entity.

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