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Collections of components
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Collection of Image components using a bag with surrogate key
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(at least, Hibernate doesn t require it). Nor do you have to declare the properties with not-null="true". They may be nullable, as can be seen in figure 6.7. We should point out that there isn t a great deal of difference between this bag mapping and a standard parent/child entity relationship like the one you map later in this chapter. The tables are identical. The choice is mainly a matter of taste. A parent/child relationship supports shared references to the child entity and true bidirectional navigation. The price you d pay is more complex lifecycles of objects. Value-typed instances can be created and associated with the persistent Item by adding a new element to the collection. They can be disassociated and permanently deleted by removing an element from the collection. If Image would be an entity class that supports shared references, you d need more code in your application for the same operations, as you ll see later. Another way to switch to a different primary key is a map. You can remove the name property from the Image class and use the image name as the key of a map:
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<map name="images" table="ITEM_IMAGE" order-by="IMAGENAME asc"> <key column="ITEM_ID"/> <map-key type="string" column="IMAGENAME"/> <composite-element class="Image"> <property name="filename" column="FILENAME" not-null="true"/> <property name="sizeX" column="SIZEX"/> <property name="sizeY" column="SIZEY"/> </composite-element> </map>
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The primary key of the collection table is now a composite of ITEM_ID and IMAGENAME. A composite element class like Image isn t limited to simple properties of basic type like filename. It may contain other components, mapped with <nestedcomposite-element>, and even <many-to-one> associations to entities. It can t
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Mapping collections and entity associations
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own collections, however. A composite element with a many-to-one association is useful, and we come back to this kind of mapping in the next chapter. This wraps up our discussion of basic collection mappings in XML. As we mentioned at the beginning of this section, mapping collections of value types with annotations is different compared with mappings in XML; at the time of writing, it isn t part of the Java Persistence standard but is available in Hibernate.
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Mapping collections with annotations
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The Hibernate Annotations package supports nonstandard annotations for the mapping of collections that contain value-typed elements, mainly org.hibernate.annotations.CollectionOfElements. Let s walk through some of the most common scenarios again.
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Basic collection mapping
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The following maps a simple collection of String elements:
@org.hibernate.annotations.CollectionOfElements( targetElement = java.lang.String.class ) @JoinTable( name = "ITEM_IMAGE", joinColumns = @JoinColumn(name = "ITEM_ID") ) @Column(name = "FILENAME", nullable = false) private Set<String> images = new HashSet<String>();
The collection table ITEM_IMAGE has two columns; together, they form the composite primary key. Hibernate can automatically detect the type of the element if you use generic collections. If you don t code with generic collections, you need to specify the element type with the targetElement attribute in the previous example it s therefore optional. To map a persistent List, add @org.hibernate.annotations.IndexColumn with an optional base for the index (default is zero):
@org.hibernate.annotations.CollectionOfElements @JoinTable( name = "ITEM_IMAGE", joinColumns = @JoinColumn(name = "ITEM_ID") ) @org.hibernate.annotations.IndexColumn( name="POSITION", base = 1
Mapping collections with annotations
) @Column(name = "FILENAME") private List<String> images = new ArrayList<String>();
If you forget the index column, this list would be treated as a bag collection, equivalent to a <bag> in XML. For collections of value types, you'd usually use <idbag> to get a surrogate primary key on the collection table. A <bag> of value typed elements doesn t really work; duplicates would be allowed at the Java level, but not in the database. On the other hand, pure bags are great for one-to-many entity associations, as you ll see in chapter 7. To map a persistent map, use @org.hibernate.annotations.MapKey:
@org.hibernate.annotations.CollectionOfElements @JoinTable( name = "ITEM_IMAGE", joinColumns = @JoinColumn(name = "ITEM_ID") ) @org.hibernate.annotations.MapKey( columns = @Column(name="IMAGENAME") ) @Column(name = "FILENAME") private Map<String, String> images = new HashMap<String, String>();
If you forget the map key, the keys of this map would be automatically mapped to the column MAPKEY. If the keys of the map are not simple strings but of an embeddable class, you can specify multiple map key columns that hold the individual properties of the embeddable component. Note that @org.hibernate.annotations.MapKey is a more powerful replacement for @javax.persistence.MapKey, which isn t very useful (see chapter 7, section 7.2.4 Mapping maps ).
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