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Advanced entity association mappings
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We now focus on many-valued entity associations, including more options for one-to-many, and finally, many-to-many mappings.
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Many-valued entity associations
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A many-valued entity association is by definition a collection of entity references. You mapped one of these in the previous chapter, section 6.4, Mapping a parent/children relationship. A parent entity instance has a collection of references to many child objects hence, one-to-many. One-to-many associations are the most important kind of entity association that involves a collection. We go so far as to discourage the use of more exotic association styles when a simple bidirectional many-to-one/one-to-many will do the job. A many-to-many association may always be represented as two many-toone associations to an intervening class. This model is usually more easily extensible, so we tend not to use many-to-many associations in applications. Also remember that you don t have to map any collection of entities, if you don t want to; you can always write an explicit query instead of direct access through iteration. If you decide to map collections of entity references, there are a few options and more complex situations that we discuss now, including a many-to-many relationship.
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One-to-many associations
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The parent/children relationship you mapped earlier was a bidirectional association, with a <one-to-many> and a <many-to-one> mapping. The many end of this association was implemented in Java with a Set; you had a collection of bids in the Item class. Let s reconsider this mapping and focus on some special cases. Considering bags It s possible to use a <bag> mapping instead of a set for a bidirectional one-to-many association. Why would you do this Bags have the most efficient performance characteristics of all the collections you can use for a bidirectional one-to-many entity association (in other words, if the collection side is inverse="true"). By default, collections in Hibernate are loaded only when they re accessed for the first time in the application. Because a bag doesn t have to maintain the index of its elements (like a list) or check for duplicate elements (like a set), you can add new elements to the bag without triggering the loading. This is an important feature if you re going to map a possibly
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Many-valued entity associations
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large collection of entity references. On the other hand, you can t eager-fetch two collections of bag type simultaneously (for example, if bids and images of an Item were one-to-many bags). We ll come back to fetching strategies in chapter 13, section 13.1, Defining the global fetch plan. In general we would say that a bag is the best inverse collection for a one-to-many association. To map a bidirectional one-to-many association as a bag, you have to replace the type of the bids collection in the Item persistent class with a Collection and an ArrayList implementation. The mapping for the association between Item and Bid is left essentially unchanged:
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<class name="Bid" table="BID"> ... <many-to-one name="item" column="ITEM_ID" class="Item" not-null="true"/> </class> <class name="Item" table="ITEM"> ... <bag name="bids" inverse="true"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </bag> </class>
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You rename the <set> element to <bag>, making no other changes. Even the tables are the same: The BID table has the ITEM_ID foreign key column. In JPA, all Collection and List properties are considered to have bag semantics, so the following is equivalent to the XML mapping:
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public class Item { ... @OneToMany(mappedBy = "item") private Collection<Bid> bids = new ArrayList<Bid>(); ... }
A bag also allows duplicate elements, which the set you mapped earlier didn t. It turns out that this isn t relevant in this case, because duplicate means you ve added a particular reference to the same Bid instance several times. You wouldn t do this
Advanced entity association mappings
in your application code. But even if you add the same reference several times to this collection, Hibernate ignores it it s mapped inverse. Unidirectional and bidirectional lists If you need a real list to hold the position of the elements in a collection, you have to store that position in an additional column. For the one-to-many mapping, this also means you should change the bids property in the Item class to List and initialize the variable with an ArrayList (or keep the Collection interface from the previous section, if you don t want to expose this behavior to a client of the class). The additional column that holds the position of a reference to a Bid instance is the BID_POSITION, in the mapping of Item:
<class name="Item" table="ITEM"> ... <list name="bids"> <key column="ITEM_ID"/> <list-index column="BID_POSITION"/> <one-to-many class="Bid"/> </list> </class>
So far this seems straightforward; you ve changed the collection mapping to <list> and added the <list-index> column BID_POSITION to the collection table (which in this case is the BID table). Verify this with the table shown in figure 7.6. This mapping isn t really complete. Consider the ITEM_ID foreign key column: It s NOT NULL (a bid has to reference an item). The first problem is that you don t specify this constraint in the mapping. Also, because this mapping is unidirectional (the collection is noninverse), you have to assume that there is no opposite side mapped to the same foreign key column (where this constraint could be declared). You need to add a not-null="true" attribute to the <key> element of the collection mapping:
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