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POJPA 2.0 retrieve entity by business key
 

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    <p>I know there have been a number of <em>similar</em> posts about this, but I couldn't find a clear answer to my problem.</p> <p>To make it as simple as possible, say I have such an entity:</p> <pre><code>@Entity public class Person implements Serializable { @Id private Long id; // PK private String name; // business key /* getters and setters */ /* override equals() and hashCode() to use the **name** field */ } </code></pre> <p>So, <code>id</code> is the PK and <code>name</code> is the business key. Say that I get a list of names, with possible duplicates, which I want to store. If I simply create one object per name, and let JPA make it persistent, my final table will contain duplicate names - Not acceptable.</p> <p>My question is what you think is the best approach, considering the alternatives I describe here below and (especially welcome) your own.</p> <h2>Possible solution 1: check the entity manager</h2> <p>Before creating a new person object, check if one with the same person name is already managed. <strong>Problem:</strong> The entity manager can only be queried by PK. IS there any workaround Idon't know about?</p> <h2>Possible solution 2: find objects by query</h2> <pre><code>Query query = em.createQuery("SELECT p FROM Person p WHERE p.name = ..."); List&lt;Person&gt; list = query.getResultList(); </code></pre> <p><strong>Questions:</strong> Should the objects requested be already loaded in the em, will this still fetch from database? If so, I suppose it would still be not very efficient if done very frequently, due to parsing the query?</p> <h2>Possible solution 3: keep a separate dictionary</h2> <p>This is possible because equals() and hashCode() are overridden to use the field <code>name</code>.</p> <pre><code>Map&lt;String,Person&gt; personDict = new HashMap&lt;String,Person&gt;(); for(String n : incomingNames) { Person p = personDict.get(n); if (p == null) { p = new Person(); p.setName(n); em.persist(p); personDict.put(n,p); } // do something with it } </code></pre> <p><strong>Problem 1:</strong> Wasting memory for large collections, as this is essentially what the entity manager does (not quite though!)</p> <p><strong>Problem 2:</strong> Suppose that I have a more complex schema, and that after the initial writing my application gets closed, started again, and needs to re-load the database. If all tables are loaded explicitly into the em, then I can easily re-populate the dictionaries (one per entity), but if I use lazy fetch and/or cascade read, then it's not so easy.</p> <p>I started recently with JPA (I use EclipseLink), so perhaps I am missing something fundamental here, because this issue seems to boil down to a very common usage pattern. </p> <p>Please enlighten me! </p>
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