NHibernate: Mapping Application Domain

Mapping Fetching Strategy

NHibernate lets you specify a default fetching strategy in the XML mapping file, with the ability to then override it at runtime in code.

NHibernate provides four different fetching strategies for any assocation: Immediate, Lazy, Eager or Batch.

Immediate Fetching

The associated object is fetched immediately, either through a sequential database read or a cache lookup.
Only an efficient fetching strategy when the associated objects are cached already.

Lazy Fetching

The associated object or collection is fetched "lazily" when the object is first accessed.
This results in a new request to the database, unless the associated object is cached.
This is the default fetching strategy configured for NHibernate 2.1.

Eager Fetching

The associated object or collection is fetched together with the owning object, using a SQL outer join, and no further database request is required.
NHibernate is limited to one eagerly fetched collection per mapped persistent class.

Batch Fetching

Used to improve the performance of lazy fetching by retrieving a batch of objects or collections when a lazy association is accessed.

The fetching strategy can be overwritten for a particular association using the outer-join attribute for an association, as follows:

For single point associations

Property Attribute	Attribute Description
outer-join="auto"	Default. Dependant on fetching strategy specified for the class. If class is configured for lazy fetching than NHibernate fetches the association lazily, otherwise NHibernate fetches the association eagerly.
outer-join="true"	NHibernate fetches the association eagerly using an outer-join, even if the class has been configured for lazy fetching.
outer-join="false"	NHibernate never fetches the association using an outer join, even when lazy fetching is disabled.

For collections

Property Attribute	Attribute Description
outer-join="true"	NHibernate fetches the collection eagerly using an outer join.
lazy="true"	NHibernate fetches the collection lazily, when it is first accessed. This option is preferred for collection mappings, and is the default in HNibernate 2.1.

The correct fetching strategy minimizes the number of SQL statements that have to be executed by lazily, eagerly, or batch-fetching objects.