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Using transitive persistence in Hibernate
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4.3.2 Cascading persistence with Hibernate
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Hibernate s transitive persistence model uses the same basic concept as persistence by reachability that is, object associations are examined to determine transitive state. However, Hibernate allows you to specify a cascade style for each association mapping, which offers more flexibility and fine-grained control for all state transitions. Hibernate reads the declared style and cascades operations to associated objects automatically. By default, Hibernate does not navigate an association when searching for transient or detached objects, so saving, deleting, or reattaching a Category won t affect the child category objects. This is the opposite of the persistence-by-reachability default behavior. If, for a particular association, you wish to enable transitive persistence, you must override this default in the mapping metadata. You can map entity associations in metadata with the following attributes:
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cascade="none", the default, tells Hibernate to ignore the association. cascade="save-update" tells Hibernate to navigate the association when the transaction is committed and when an object is passed to save() or update() and save newly instantiated transient instances and persist changes to
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detached instances.
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cascade="delete" tells Hibernate to navigate the association and delete persistent instances when an object is passed to delete(). cascade="all" means to cascade both save-update and delete, as well as calls to evict and lock. cascade="all-delete-orphan" means the same as cascade="all" but, in addi-
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tion, Hibernate deletes any persistent entity instance that has been removed (dereferenced) from the association (for example, from a collection).
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cascade="delete-orphan" Hibernate will delete any persistent entity
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instance that has been removed (dereferenced) from the association (for example, from a collection). This association-level cascade style model is both richer and less safe than persistence by reachability. Hibernate doesn t make the same strong guarantees of referential integrity that persistence by reachability provides. Instead, Hibernate partially delegates referential integrity concerns to the foreign key constraints of the underlying relational database. Of course, there is a good reason for this design decision: It allows Hibernate applications to use detached objects efficiently, because you can control reattachment of a detached object graph at the association level.
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Working with persistent objects
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Let s elaborate on the cascading concept with some example association mappings. We recommend that you read the next section in one turn, because each example builds on the previous one. Our first example is straightforward; it lets you save newly added categories efficiently.
4.3.3 Managing auction categories
System administrators can create new categories, rename categories, and move subcategories around in the category hierarchy. This structure can be seen in figure 4.3. Now, we map this class and the association:
<class name="Category" table="CATEGORY"> ... <property name="name" column="CATEGORY_NAME"/> <many-to-one name="parentCategory" class="Category" column="PARENT_CATEGORY_ID" cascade="none"/> <set name="childCategories" table="CATEGORY" cascade="save-update" inverse="true"> <key column="PARENT_CATEGORY_ID"/> <one-to-many class="Category"/> </set> ... </class>
Category
0..* name : String
Figure 4.3 Category class with association to itself
This is a recursive, bidirectional, one-to-many association, as briefly discussed in chapter 3. The one-valued end is mapped with the <many-to-one> element and the Set typed property with the <set>. Both refer to the same foreign key column: PARENT_CATEGORY_ID. Suppose we create a new Category as a child category of Computer (see figure 4.4). We have several ways to create this new Laptops object and save it in the database. We could go back to the database and retrieve the Computer category to which our new Laptops category will belong, add the new category, and commit the transaction:
Using transitive persistence in Hibernate
Electronics : Category
Cell Phones : Category
Computer : Category
Desktop PCs : Category
Monitors : Category
Laptops : Category
Adding a new Category to the object graph
Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); Category computer = (Category) session.get(Category.class, computerId); Category laptops = new Category("Laptops"); computer.getChildCategories().add(laptops); laptops.setParentCategory(computer); tx.commit(); session.close();
The computer instance is persistent (attached to a session), and the childCategories association has cascade save enabled. Hence, this code results in the new laptops category becoming persistent when tx.commit() is called, because Hibernate cascades the dirty-checking operation to the children of computer. Hibernate executes an INSERT statement. Let s do the same thing again, but this time create the link between Computer and Laptops outside of any transaction (in a real application, it s useful to manipulate an object graph in a presentation tier for example, before passing the graph back to the persistence layer to make the changes persistent):
Category computer = ... // Loaded in a previous session Category laptops = new Category("Laptops"); computer.getChildCategories().add(laptops); laptops.setParentCategory(computer);
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