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POHow to store Key-Value pairs using JDO (datanucleus & appengine)
 

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  1. POHow to store Key-Value pairs using JDO (datanucleus & appengine)
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    <p>I have defined a ConfigurationProperty class, which is basicly a key-value pair (with key being a strictly positive integer and value being a string):</p> <pre><code>import javax.jdo.annotations.PersistenceCapable; import javax.jdo.annotations.Persistent; import javax.jdo.annotations.PrimaryKey; @PersistenceCapable(detachable = "true") public final class ConfigurationProperty { @PrimaryKey @Persistent private Integer id; @Persistent private String value; public ConfigurationProperty(Integer id, String value) { this.id = id; this.setValue(value); } public Integer getId() { return this.id; } public String getValue() { return value; } public void setValue(String value) { this.value = value; } } </code></pre> <p>As you can see, I'm defining a field "id" (which is the key of the key-value pair) and use it as the class's primary key.</p> <p>However, when I try to access an entry:</p> <pre><code>int searchId = 4; ConfigurationProperty property = this.persistence.getObjectById(ConfigurationProperty.class, searchId); </code></pre> <p>I get the exception:</p> <pre><code>javax.jdo.JDOFatalUserException: Received a request to find an object of type [mypackage].ConfigurationProperty identified by 4. This is not a valid representation of a primary key for an instance of [mypackage].ConfigurationProperty. NestedThrowables: org.datanucleus.exceptions.NucleusFatalUserException: Received a request to find an object of type [mypackage].ConfigurationProperty identified by 4. This is not a valid representation of a primary key for an instance of [mypackage].ConfigurationProperty.) </code></pre> <p>I'm almost sure I would not run into an exception should I create a separate field for the primary key and the pair's key, but that would create some form of redundancy and possibly performance issues, since the pair's key <em>is</em> unique.</p> <p>I also only get the exception on Google Appengine. When I tested the application using a Derby database, there were no problems.</p> <p>Thank you for any suggestion!</p>
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