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Many-valued entity associations
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Many-to-many entity association mapped to an association table
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In Hibernate XML you can also switch to an <idbag> with a separate primary key column on the join table:
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<idbag name="items" table="CATEGORY_ITEM cascade="save-update"> <collection-id type="long" column="CATEGORY_ITEM_ID"> <generator class="sequence"/> </collection-id> <key column="CATEGORY_ID"/> <many-to-many class="Item" column="ITEM_ID"/> </idbag>
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As usual with an <idbag> mapping, the primary key is a surrogate key column, CATEGORY_ITEM_ID. Duplicate links are therefore allowed; the same Item can be added twice to a Category. (This doesn t seem to be a useful feature.) With annotations, you can switch to an identifier bag with the Hibernate @CollectionId:
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@ManyToMany @CollectionId( columns = @Column(name = "CATEGORY_ITEM_ID"), type = @org.hibernate.annotations.Type(type = "long"), generator = "sequence" ) @JoinTable( name = "CATEGORY_ITEM", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")} ) private Collection<Item> items = new ArrayList<Item>();
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A JPA XML descriptor for a regular many-to-many mapping with a set (you can t use a Hibernate extension for identifier bags) looks like this:
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Advanced entity association mappings
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<entity class="auction.model.Category" access="FIELD"> ... <many-to-many name="items"> <join-table name="CATEGORY_ITEM"> <join-column name="CATEGORY_ID"/> <inverse-join-column name="ITEM_ID"/> </join-table> </many-to-many> </entity>
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You may even switch to an indexed collection (a map or list) in a many-to-many association. The following example maps a list in Hibernate XML:
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<list name="items" table="CATEGORY_ITEM" cascade="save-update"> <key column="CATEGORY_ID"/> <list-index column="DISPLAY_POSITION"/> <many-to-many class="Item" column="ITEM_ID"/> </list>
The primary key of the link table is a composite of the CATEGORY_ID and DISPLAY_POSITION columns; this mapping guarantees that the position of each Item in a Category is persistent. Or, with annotations:
@ManyToMany @JoinTable( name = "CATEGORY_ITEM", joinColumns = {@JoinColumn(name = "CATEGORY_ID")}, inverseJoinColumns = {@JoinColumn(name = "ITEM_ID")} ) @org.hibernate.annotations.IndexColumn(name = "DISPLAY_POSITION") private List<Item> items = new ArrayList<Item>();
As discussed earlier, JPA only supports ordered collections (with an optional @OrderBy annotation or ordered by primary key), so you again have to use a Hibernate extension for indexed collection support. If you don t add an @IndexColumn, the List is stored with bag semantics (no guaranteed persistent order of elements). Creating a link between a Category and an Item is easy:
aCategory.getItems().add(anItem);
Bidirectional many-to-many associations are slightly more difficult. A bidirectional many-to-many association You know that one side in a bidirectional association has to be mapped as inverse because you have named the foreign key column(s) twice. The same principle
Many-valued entity associations
applies to bidirectional many-to-many associations: Each row of the link table is represented by two collection elements, one element at each end of the association. An association between an Item and a Category is represented in memory by the Item instance in the items collection of the Category, but also by the Category instance in the categories collection of the Item. Before we discuss the mapping of this bidirectional case, you have to be aware that the code to create the object association also changes:
aCategory.getItems().add(anItem); anItem.getCategories().add(aCategory);
As always, a bidirectional association (no matter of what multiplicity) requires that you set both ends of the association. When you map a bidirectional many-to-many association, you must declare one end of the association using inverse="true" to define which side s state is used to update the join table. You can choose which side should be inverse. Recall this mapping of the items collection from the previous section:
<class name="Category" table="CATEGORY"> ... <set name="items" table="CATEGORY_ITEM" cascade="save-update"> <key column="CATEGORY_ID"/> <many-to-many class="Item" column="ITEM_ID"/> </set>
You may reuse this mapping for the Category end of the bidirectional association and map the other side as follows:
<class name="Item" table="ITEM"> ... <set name="categories" table="CATEGORY_ITEM inverse="true" cascade="save-update"> <key column="ITEM_ID"/> <many-to-many class="Category" column="CATEGORY_ID"/> </set> </class>
Note the inverse="true". Again, this setting tells Hibernate to ignore changes made to the categories collection and that the other end of the association, the items collection, is the representation that should be synchronized with the database if you link instances in Java code.
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