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<set name="Bids" lazy="true"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set>
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You can even enable batch fetching for the collection. In this case, the batch size doesn t refer to the number of bids in the batch; it refers to the number of collections of bids:
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<set name="Bids" lazy="true" batch-size="9"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set>
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This mapping tells NHibernate to load up to nine collections of bids in one batch, depending on how many uninitialized collections of bids are currently present in the items associated with the session. In other words, if five Item instances have persistent state in an ISession, and all have an uninitialized Bids collection, NHibernate will automatically load all five collections in a single SQL query if one is accessed. If there
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Working with persistent objects
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are 11 items, only 9 collections will be fetched. Batch fetching can significantly reduce the number of queries required for hierarchies of objects (for example, when loading the tree of parent and child Category objects). Let s talk about a special case: many-to-many associations (we discuss this mapping in more detail in chapter 7). You usually use a link table (some developers also call it relationship table or association table ) that holds only the key values of the two associated tables and therefore allows a many-to-many multiplicity. This additional table must be considered if you decide to use eager fetching. Look at the following straightforward many-to-many example, which maps the association from Category to Item:
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<set name="Items" outer-join="true" table="CATEGORY_ITEM"> <key column="CATEGORY_ID"/> <many-to-many column="ITEM_ID" class="Item"/> </set>
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In this case, the eager fetching strategy refers only to the association table CATEGORY_ITEM. If you load a Category with this fetching strategy, NHibernate automatically fetches all link entries from CATEGORY_ITEM in a single outer join SQL query, but not the item instances from ITEM! The entities contained in the many-to-many association can also be fetched eagerly with the same SQL query. The <many-to-many> element lets you customize this behavior:
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<set name="Items" outer-join="true" table="CATEGORY_ITEM"> <key column="CATEGORY_ID"/> <many-to-many column="ITEM_ID" outer-join="true" class="Item"/> </set>
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NHibernate now fetches all Items in a Category with a single outer join query when
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the Category is loaded. But keep in mind that we usually recommend lazy loading as the default fetching strategy and that NHibernate is limited to one eagerly fetched collection per mapped persistent class.
SETTING THE FETCH DEPTH
We now discuss a global fetching strategy setting: the maximum fetch depth. This setting controls the number of outer-joined tables NHibernate uses in a single SQL query. Consider the complete association chain from Category to Item, and from Item to Bid. The first is a many-to-many association, and the second is one-to-many; hence both associations are mapped with collection elements. If you declare outerjoin="true" for both associations (don t forget the special <many-to-many> declaration) and load a single Category, how many queries will NHibernate execute Will only the Items be eagerly fetched, or also all the Bids of each Item You probably expect a single query with an outer join operation including the CATEGORY, CATEGORY_ITEM, ITEM, and BID tables. But this isn t the case by default. NHibernate s outer join fetch behavior is controlled with the global configuration option hibernate.max_fetch_depth. If you set this to 1 (also the default), NHibernate fetches only the Category and the link entries from the CATEGORY_ITEM association table. If you set it to 2, NHibernate executes an outer join that also includes the Items in the same SQL query. Setting this option to 3 won t, as you might have
Retrieving objects
expected, also include the bids of each item in the same SQL query. The limitation to one outer joined collection applies here, preventing slow Cartesian products. Recommended values for the fetch depth depend on the join performance and the size of the database tables; test your applications with low values (less than 4) first, and decrease or increase the number while tuning your application. The global maximum fetch depth also applies to single-ended association (<many-to-one>, <one-toone>) mapped with an eager fetching strategy or using the auto default. Keep in mind that eager fetching strategies declared in the mapping metadata are effective only if you use retrieval by identifier, use the criteria query API, or navigate through the object graph manually. Any HQL query may specify its own fetching strategy at runtime, thus ignoring the mapping defaults. You can also override the defaults (that is, not ignore them) with criteria queries. This is an important difference, and we cover it in more detail in section 8.3.2. But you may sometimes want to initialize a proxy or a collection wrapper manually with a simple API call.
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