Advanced mapping concepts in Java

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Advanced mapping concepts
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<many-to-many class="Item" column="ITEM_ID"/> </set> </class>
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We can reuse this mapping for the Category end of the bidirectional association. We map the Item end as follows:
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<class name="Item" table="ITEM"> ... <set name="categories" table="CATEGORY_ITEM" lazy="true" inverse="true" cascade="save-update"> <key column="ITEM_ID"/> <many-to-many class="Item" column="CATEGORY_ID"/> </set> </class>
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Note the use of inverse="true". Once again, this setting tells Hibernate to ignore changes made to the categories collection and use the other end of the association (the items collection) as the representation that should be synchronized with the database if we manipulate the association in Java code. We ve chosen cascade="save-update" for both ends of the collection; this isn t unreasonable. On the other hand, cascade="all", cascade="delete", and cascade="all-delete-orphans" aren t meaningful for many-to-many associations, since an instance with potentially many parents shouldn t be deleted when just one parent is deleted. What kinds of collections may be used for bidirectional many-to-many associations Do you need to use the same type of collection at each end It s reasonable to use, for example, a list at the end not marked inverse="true" (or explicitly set false) and a bag at the end that is marked inverse="true". You can use any of the mappings we ve shown for unidirectional many-to-many associations for the noninverse end of the bidirectional association. <set>, <idbag>, <list>, and <map> are all possible, and the mappings are identical to those shown previously. For the inverse end, <set> is acceptable, as is the following bag mapping:
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<class name="Item" table="ITEM"> ... <bag name="categories" table="CATEGORY_ITEM lazy="true" inverse="true" cascade="save-update">
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Mapping entity associations
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<key column="ITEM_ID"/> <many-to-many class="Item" column="CATEGORY_ID"/> </bag> </class>
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This is the first time we ve shown the <bag> declaration: It s similar to an <idbag> mapping, but it doesn t involve a surrogate key column. It lets you use a List (with bag semantics) in a persistent class instead of a Set. Thus it s preferred if the noninverse side of a many-to-many association mapping is using a map, list, or bag (which all permit duplicates). Remember that a bag doesn t preserve the order of elements, despite the List type in the Java property definition. No other mappings should be used for the inverse end of a many-to-many association. Indexed collections (lists and maps) can t be used, since Hibernate won t initialize or maintain the index column if inverse="true". This is also true and important to remember for all other association mappings involving collections: an indexed collection (or even arrays) can t be set to inverse="true". We already frowned at the use of a many-to-many association and suggested the use of composite element mappings as an alternative. Let s see how this works. Using a collection of components for a many-to-many association Suppose we need to record some information each time we add an Item to a Category. For example, we might need to store the date and the name of the user who added the item to this category. We need a Java class to represent this information:
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public class CategorizedItem { private String username; private Date dateAdded; private Item item; private Category category; .... }
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(We omitted the accessors and equals() and hashCode() methods, but they would be necessary for this component class.) We map the items collection on Category as follows:
<set name="items" lazy="true" table="CATEGORY_ITEMS"> <key column="CATEGORY_ID"/> <composite-element class="CategorizedItem"> <parent name="category"/> <many-to-one name="item" class="Item" column="ITEM_ID" not-null="true"/> <property name="username" column="USERNAME" not-null="true"/>
Advanced mapping concepts
<property name="dateAdded" column="DATE_ADDED" not-null="true"/> </composite-element> </set>
We use the <many-to-one> element to declare the association to Item, and we use the <property> mappings to declare the extra association-related information. The link table now has four columns: CATEGORY_ID, ITEM_ID, USERNAME, and DATE_ADDED. The columns of the CategorizedItem properties should never be null: otherwise we can t identify a single link entry, because they re all part of the table s primary key. You can see the table structure in figure 6.11. In fact, rather than mapping just the username, we might like to keep an actual reference to the User object. In this case, we have the following ternary association mapping:
<set name="items" lazy="true" table="CATEGORY_ITEMS"> <key column="CATEGORY_ID"/> <composite-element class="CategorizedItem"> <parent name="category"/> <many-to-one name="item" class="Item" column="ITEM_ID" not-null="true"/> <many-to-one name="user" class="User" column="USER_ID" not-null="true"/> <property name="dateAdded" column="DATE_ADDED" not-null="true"/> </composite-element> </set>
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