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POHow to use enums with JPA
 

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    <p>I have an existing database of a film rental system. Each film has a has a rating attribute. In SQL they used a constraint to limit the allowed values of this attribute.</p> <pre><code>CONSTRAINT film_rating_check CHECK ((((((((rating)::text = ''::text) OR ((rating)::text = 'G'::text)) OR ((rating)::text = 'PG'::text)) OR ((rating)::text = 'PG-13'::text)) OR ((rating)::text = 'R'::text)) OR ((rating)::text = 'NC-17'::text))) </code></pre> <p>I think it would be nice to use a Java enum to map the constraint into the object world. But it's not possible to simply take the allowed values because of the special char in "PG-13" and "NC-17". So I implemented the following enum:</p> <pre><code>public enum Rating { UNRATED ( "" ), G ( "G" ), PG ( "PG" ), PG13 ( "PG-13" ), R ( "R" ), NC17 ( "NC-17" ); private String rating; private Rating(String rating) { this.rating = rating; } @Override public String toString() { return rating; } } @Entity public class Film { .. @Enumerated(EnumType.STRING) private Rating rating; .. </code></pre> <p>With the toString() method the direction enum -> String works fine, but String -> enum does not work. I get the following exception:</p> <blockquote> <p>[TopLink Warning]: 2008.12.09 01:30:57.434--ServerSession(4729123)--Exception [TOPLINK-116] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.DescriptorException Exception Description: No conversion value provided for the value [NC-17] in field [FILM.RATING]. Mapping: oracle.toplink.essentials.mappings.DirectToFieldMapping[rating-->FILM.RATING] Descriptor: RelationalDescriptor(de.fhw.nsdb.entities.Film --> [DatabaseTable(FILM)])</p> </blockquote> <p>cheers </p> <p>timo</p>
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    1. COdid you omit the @Column attribute for the field intentionally? Without it, it would be hard to persist anything...
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    2. COYou need a static hashmap of your enums inside your enum, and a static method "getByRating"
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