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POHwo to configure a JDO transaction to simulate creating a database sequence in App Engine Java?
 

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  1. POHwo to configure a JDO transaction to simulate creating a database sequence in App Engine Java?
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    <p>This syntax does not produce unique values across different Fetch Groups:</p> <pre><code>@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) private long id; </code></pre> <p>So <em>I've written the method below to simulate generating a database sequence for my Order model</em>. But what I'm not sure about is what the transactional state needs to be configured as here (<strong><em><code>isolation level / nontransactional read / nontransactional write / optimistic / etc.</code></em>)</strong>:</p> <pre><code>public long getNextId() { PersistenceManager pm = this.getPm(); Transaction tx = pm.currentTransaction(); tx.begin(); long nextId = 0; Query query = pm.newQuery("select id from orders order by id desc"); query.setRange(0, 1); try { List&lt;Order&gt; results = (List&lt;Order&gt;) query.execute(); if (results.iterator().hasNext()) { for (Order r : results) { nextId = r.getId() + 1; } } else { return 0; } } catch (Exception e) { return 0; } tx.commit(); return nextId; } </code></pre> <p>Does the scope of the transaction need to be broader than just this method? In other words, should it also include the insert action for the new Order?</p> <p><strong>I want to make sure that no two Orders that I insert can have the same id value across the entire application.</strong></p>
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