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POmixing joined and single table inheritance and querying for all objects
 

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    <p>I have a webapp that is working with the following configuration (I changed the entity names):</p> <pre><code>@Entity @Inheritance(strategy = InheritanceType.JOINED) @Table(name = "animals") public abstract class Animal { ... @MappedSuperclass @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name = "type") public abstract class Mammal extends Animal { ... @Entity @Table(name = "mammals") @PrimaryKeyJoinColumn(name = "mammal_id") @DiscriminatorValue(value = "dog") public class Dog extends Mammal { ... @Entity @Table(name = "mammals") @PrimaryKeyJoinColumn(name = "mammal_id") @DiscriminatorValue(value = "cat") public class Cat extends Mammal { ... </code></pre> <p>So I have 2 tables: animals and mammals. This configuration is working, unless I don't completely understand how. The inheritance type is changed from JOINED to SINGLE_TABLE in Mammal, so cats and dogs are stored in mammals table joined with animals table by mammal_id column.</p> <p>The problem is that is not possible to retrieve a mammal (cat or dog) in a polymorphic way:</p> <pre><code>Mammal mammal = (Mammal) hibernateTemplate.get(Mammal.class, id); </code></pre> <p>or a list of mammals (dogs and cats) with a single Hibernate query:</p> <pre><code>List&lt;Mammal&gt; list = (List&lt;Mammal&gt;) hibernateTemplate.find("from Mammal"); </code></pre> <p>I get: "Unknown entity: Mammal", because Mammal is not an entity, it is just a mapped super class.</p> <p>I tried several changes in configuration for example the one below. This seems to make sense, but in that case I got "Could not load entity: Mammal" because it looks for tables cats and dogs (which doesn't exist).</p> <pre><code>@Entity @Inheritance(strategy = InheritanceType.JOINED) @Table(name = "animals") public abstract class Animal { ... @Entity @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @Table(name = "mammals") @PrimaryKeyJoinColumn(name = "mammal_id") @DiscriminatorColumn(name = "type") public abstract class Mammal extends Animal { ... @Entity @DiscriminatorValue(value = "dog") public class Dog extends Mammal { ... @Entity @DiscriminatorValue(value = "cat") public class Cat extends Mammal { ... </code></pre> <p>So, to summarize: I want to keep the webapp working the same way (2 tables: animals and mammals) but at the same time be able to do a polymorphic query: retrieve a list of mammals (cats and dogs) with one query.</p> <p>I hope I was clear enough and thanks in advance for any help.</p> <p>ANOTHER TRY:</p> <p>"Foreign key circularity dependency involving the following tables: "</p> <pre><code>@Entity @Inheritance(strategy = InheritanceType.JOINED) @Table(name = "animals") public abstract class Animal { ... @Entity @Table(name = "mammals") @PrimaryKeyJoinColumn(name = "mammal_id") @Inheritance(strategy = InheritanceType.SINGLE_TABLE) @DiscriminatorColumn(name = "type") public abstract class Mammal extends Animal { ... @Entity @Table(name = "mammals") @DiscriminatorValue(value = "dog") public class Dog extends Mammal { ... @Entity @Table(name = "mammals") @DiscriminatorValue(value = "cat") public class Cat extends Mammal { ... </code></pre>
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