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Handling special kinds of data
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If a class with a natural key does not declare a version or timestamp property, it s more difficult to get saveOrUpdate() and cascades to work correctly. You might use a custom Hibernate Interceptor as discussed later in this chapter. (On the other hand, if you re happy to use explicit save() and explicit update() instead of saveOrUpdate() and cascades, Hibernate doesn t need to be able to distinguish between transient and detached instances; so, you can safely ignore this advice.) Composite natural keys extend the same ideas. Mapping a table with a composite key As far as Hibernate is concerned, a composite key may be handled as an assigned identifier of value type (the Hibernate type is a component). Suppose the primary key of our user table consisted of a USERNAME and an ORGANIZATION_ID. We could add a property named organizationId to the User class and use the following mapping:
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<class name="User" table="USER"> <composite-id> <key-property name="username" column="USERNAME"/> <key-property name="organizationId" column="ORGANIZATION_ID"/> </composite-id> <version name="version" column="VERSION" unsaved-value="0"/> ... </class>
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The code to save a new User would look like this:
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User user = new User(); // Assign a primary key value user.setUsername("john"); user.setOrganizationId(42); // Set property values user.setFirstname("John"); user.setLastname("Doe"); session.saveOrUpdate(user); // will save, since version is 0 session.flush();
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But what object could we use as the identifier when we called load() or get() It s possible to use an instance of the User; for example:
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User user = new User(); // Assign a primary key value user.setUsername("john"); user.setOrganizationId(42); // Load the persistent state into user session.load(User.class, user);
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In this code snippet, User acts as its own identifier class. Note that we now have to implement Serializable and equals()/hashCode() for this class. It s much more elegant to define a separate composite identifier class that declares just the key properties. We call this class UserId:
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public class UserId extends Serializable { private String username; private String organizationId; public UserId(String username, String organizationId) { this.username = username; this.organizationId = organizationId; } // Getters... public boolean equals(Object o) { if (this == o) return true; if (o = null) return false; if (!(o instanceof UserId)) return false; final UserId userId = (UserId) o; if (!organizationId.equals(userId.getOrganizationId())) return false; if (!username.equals(userId.getUsername())) return false; return true; } public int hashCode() { return username.hashCode(); ) }
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It s critical that we implement equals() and hashCode() correctly, since Hibernate uses these methods to do cache lookups. Composite key classes are also expected to implement Serializable. Now, we d remove the userName and organizationId properties from User and add a userId property. We d use the following mapping:
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<class name="User" table="USER"> <composite-id name="userId" class="UserId"> <key-property name="userName" column="USERNAME"/> <key-property name="organizationId" column="ORGANIZATION_ID"/> </composite-id> <version name="version" column="VERSION" unsaved-value="0"/> ... </class>
We could save a new instance using this code:
UserId id = new UserId("john", 42); User user = new User(); // Assign a primary key value user.setUserId(id); // Set property values user.setFirstname("John"); user.setLastname("Doe"); session.saveOrUpdate(user); // will save, since version is 0 session.flush();
The following code shows how to load an instance:
UserId id = new UserId("john", 42); User user = (User) session.load(User.class, id);
Now, suppose the ORGANIZATION_ID was a foreign key to the ORGANIZATION table, and that we wished to represent this association in our Java model. Our recommended way to do this is to use a <many-to-one> association mapped with insert="false" update="false", as follows:
<class name="User" table="USER"> <composite-id name="userId" class="UserId"> <key-property name="userName" column="USERNAME"/> <key-property name="organizationId" column="ORGANIZATION_ID"/> </composite-id>
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<version name="version" column="VERSION" unsaved-value="0"/> <many-to-one name="organization" class="Organization" column="ORGANIZATION_ID" insert="false" update="false"/> ... </class>
This use of insert="false" update="false" tells Hibernate to ignore that property when updating or inserting a User, but we may of course read it with john.getOrganization(). An alternative approach is to use a <key-many-to-one>:
<class name="User" table="USER"> <composite-id name="userId" class="UserId"> <key-property name="userName" column="USERNAME"/> <key-many-to-one name="organization" class="Organization" column="ORGANIZATION_ID"/> </composite-id> <version name="version" column="VERSION" unsaved-value="0"/> ... </class>
However, it s usually inconvenient to have an association in a composite identifier class, so this approach isn t recommended except in special circumstances. Since USER has a composite primary key, any referencing foreign key is also composite. For example, the association from Item to User (the seller) is now mapped to a composite foreign key. To our relief, Hibernate can hide this detail from the Java code. We can use the following association mapping for Item:
<many-to-one name="seller" class="User"> <column name="USERNAME"/> <column name="ORGANIZATION_ID"/> </many-to-one>
Any collection owned by the User class will also have a composite foreign key for example, the inverse association, items, sold by this user:
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