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copied!<p>Your mapping is totally valid and works with EclipseLink as JPA 2.0 implementation (without the <code>Fetch</code> annotation of course), but indeed fails with Hibernate. </p> <p>Here is the DDL with Hibernate:</p> <pre><code>create table foo_bar_map (foo_id bigint not null, bar_id bigint not null, order_index integer not null, primary key (foo_id, order_index), unique (bar_id)) alter table foo_bar_map add constraint FK14F1CB7FA042E82 foreign key (bar_id) references Bar4022509 alter table foo_bar_map add constraint FK14F1CB7B6DBCCDC foreign key (foo_id) references Foo4022509 </code></pre> <p>So let's say <code>Foo#1</code> holds a list with <code>Bar#1</code>, <code>Bar#2</code>, <code>Bar#3</code>, the join table contains:</p> <pre><code>foo_id | bar_id | order_index 1 | 1 | 1 1 | 2 | 2 1 | 3 | 3 </code></pre> <p>When removing, say the first item from the list, Hibernate first <code>delete</code> the last row (WTF?) from the join table:</p> <pre><code>foo_id | bar_id | order_index 1 | 1 | 1 1 | 2 | 2 </code></pre> <p>And then tries to <code>update</code> the <code>bar_id</code> column in the join table instead of the <code>order_index</code> (WTF!?) to reflect the "new" ordering of the items in the list. First (schematically):</p> <pre><code>foo_id | bar_id | order_index 1 | 2 | 1 1 | 2 | 2 </code></pre> <p>where the next step would result in: </p> <pre><code>foo_id | bar_id | order_index 1 | 2 | 1 1 | 3 | 2 </code></pre> <p>Obviously, this approach doesn't sound right and <strong>doesn't work</strong> because of the <code>unique</code> constraint on <code>bar_id</code>. More generally, why the hell does Hibernate mess with the <code>bar_id</code> instead of updating the <code>order_index</code> column? </p> <p>I consider this to be an Hibernate bug (reported as <s><a href="https://hibernate.atlassian.net/browse/HHH-5694" rel="nofollow noreferrer">HHH-5694</a></s>, see <a href="https://hibernate.atlassian.net/browse/HHH-1268" rel="nofollow noreferrer">HHH-1268</a> now).</p>

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