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POJPA 2: Determine an entitys id by looking up JoinColumn - Shared primary key in one-to-one relation
 

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  1. POJPA 2: Determine an entitys id by looking up JoinColumn - Shared primary key in one-to-one relation
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    <p>I have two entity classes Client and Preferences in a bidirectional one-to-one relation. The entities should share the same primary key, which is generated programmatically by java.util.UUID.</p> <p>Here is the minimalistic scenario:</p> <pre class="lang-java prettyprint-override"><code>@Entity public class Client implements Serializable { @Id private String id; @OneToOne(mappedBy = "client", casacade = CascadeType.PERSIST) private Preferences preferences; // ... } @Entity public class Preferences implements Serializable { @Id @OneToOne @JoinColumn private Client client; // ... } </code></pre> <p>Everything works fine but I am disaffected because I can't find a clean way to provide a simple id field/property in Preferences.</p> <p>In the scenario above I am forced to determine it by calling <code>client.getId();</code> all the time which looks quite messed up. So I was trying to figure out some alternatives. Here is what I found out so far:</p> <p>Alternative A: Persist a basic value called id:</p> <pre class="lang-java prettyprint-override"><code>@Entity public class Preferences implements Serializable { @Id @OneToOne @JoinColumn private Client client; private String id; public setClient (Client client) { this.client = client; id = client.getId(); } // ... } </code></pre> <p>The main problem with that solution is that it would store the same value twice...why then share a pk at all?!</p> <p>Alternative B: Transient id:</p> <pre class="lang-java prettyprint-override"><code>@Entity public class Preferences implements Serializable { @Id @OneToOne @JoinColumn private Client client; @Transient private String id; @PostLoad private void postLoad () { id = client.getId(); } public setClient (Client client) { this.client = client; id = client.getId(); } // ... } </code></pre> <p>Okay, here we don't save redundant data but Client must be fetched eagerly erverytime. That's the default behaviour in one-to-one relations but what if it would be better someday to have that client being fetched lazily? I don't like my options being restricted for no reason. </p> <p>Is there a way to read the id given by Client directly out of that JoinColumn or should I use solution A/B?</p> <p>I'm trying to stick to the spec, provider independent hints would be nice.</p> <p>Thank you!</p>
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