Defining Class Mappings

Chapter 11 Generating .NET 2.0 Persistent Objects and Windows Applications

Defining Class Mappings

There are two kinds of classes in NHibernate:

Entity classes - have their own database identities, mapping files and life cycles
Value type classes - depend on entity classes. Also known as component classes

NHibernate uses mapping XML files to define the mapping metadata. Each mapping file can contain metadata for one or many classes. PowerDesigner uses the following grouping strategy:

A separate mapping file is generated for each single entity class that is not in an inheritance hierarchy.
A separate mapping file is generated for each inheritance hierarchy that has a unique mapping strategy. All mappings of subclasses are defined in the mapping file. The mapping file is generated for the root class of the hierarchy.
No mapping file is generated for a single value type class that is not in an inheritance hierarchy. Its mapping is defined in its owner's mapping file.

Classes can be mapped to tables or views. Since views have many constraints and limited functionality (for example they do not have primary keys and reference keys), some views cannot be updated, and the mappings may not work properly in some cases.

There are some conditions that need to be met in order to generate mapping for a specific class:

The source can be generated. This may not be possible if, for example, the visibility of the class is private.
The class is persistent.
The generation mode is not set to Generate ADT (abstract data type) and Value Type.
If the class is an inner class, it must be static, and have public visibility. NHibernate should then be able to create instances of the class.

NHibernate specific class mapping options are defined in the NHibernate tab of the class property sheet:

Option	Description
Dynamic insert	Specifies that INSERT SQL should be generated at runtime and will contain only the columns whose values are not null. NHibernate Tag: dynamic-insert
Dynamic update	Specifies that UPDATE SQL should be generated at runtime and will contain only the columns whose values have changed. NHibernate Tag: dynamic-update
Select before update	Specifies that NHibernate should never perform a SQL UPDATE unless it is certain that an object is actually modified. NHibernate Tag: select-before-update
Default cascade	Specifies the default cascade style. NHibernate Tag: default-cascade
Default access	Specifies the default access type (field or property) NHibernate Tag: default-access
Proxy name	Specifies an interface to use for lazy initializing proxies. NHibernate Tag: proxy
Batch size	Specifies a "batch size" for fetching instances of this class by identifier. NHibernate Tag: batch-size
Check	Specifies a SQL expression used to generate a multi-row check constraint for automatic schema generation. NHibernate Tag: check
Polymorphism	Specifies whether implicit or explicit query polymorphism is used. NHibernate Tag: polymorphism
Schema name	Specifies the name of the database schema. NHibernate Tag: schema
Catalog name	Specifies the name of the database catalog. NHibernate Tag: catalog
Row id	Specifies that NHibernate can use the ROWID column on databases which support it (for example, Oracle). NHibernate Tag: rowed
Persister class name	Specifies a custom persistence class. NHibernate Tag: persister
Lazy	Specifies that the class should be lazy fetching. NHibernate Tag: lazy
Mutable	Specifies that instances of the class are mutable. NHibernate Tag: mutable
Abstract class	Specifies that the class is abstract. NHibernate Tag: abstract
Auto import	Specifies that an unqualified class name can be used in a query NHibernate Tag: Auto-import
Discriminator column	Specifies the discriminator column or formula for polymorphic behavior in a one table per hierarchy mapping strategy. NHibernate Tag: discriminator
Discriminator value	Specifies a value that distinguishes individual subclasses, which are used for polymorphic behavior. NHibernate Tag: discriminator-value
Discriminator type	Specifies the discriminator type. NHibernate Tag: type
Force usage of discriminator	Forces NHibernate to specify allowed discriminator values even when retrieving all instances of the root class. NHibernate Tag: force
Do not use discriminator in insert	Forces NHibernate to not include the column in SQL INSERTs NHibernate Tag: insert
Optimistic lock type	Specifies an optimistic locking strategy. NHibernate Tag: optimistic-lock
Optimistic lock column name	Specifies the column used for optimistic locking. A field is also generated if this option is set. NHibernate Tag: version/ timestamp
Optimistic lock unsaved value	Specifies whether an unsaved value is null or undefined. NHibernate Tag: unsaved-value