static void

NHibernate Mapping

See simple examples. Add the .hbm.xml files as "Embedded Resource". The number one error!
You can add xml intellisense by copying the nhibernate-mapping.xsd file to %ProgramFiles%/Microsoft Visual Studio 10.0\Xml\Schemas.

Mapping to tables

• The easiest is table per concrete class- like Linq2Sql
• For Table per class hierarchy (simple polymorphic entities) use a discriminator column in the sql table
  Add <subclass ... discriminator-value="A"> ... <discriminator column="TYPE" />
  You could also use sql: formula="case when TYPE is Null Then 'A' Else 'B' end"
• Table per subclass- the base class is a table, and there are tables for each subclass whose primary key is a foreign key to the base table. NHibernate selects with an outer-join.
  Use <joined-subclass table="SubTable"><key column="ID"/><property ...

Identity

generator can be:

native (or identity or sequence)

Integer auto-assigned primary keys - Identity columns in SQLServer, sequences in Oracle.
Entities are not just saved at Flush/Commit.

guid or guid.comb

NHibernate assigns a guid (guid.comb uses datetime to make sequential ids).

hilo

Read unique keys from table (eg hibernate_unique_key).

assigned

Natural keys. New entities must be saved with ISession.Save, not SaveOrUpdate.

Composite keys - use <composite-id class="KeyClass"> <key-property ... (or key-many-to-one for FK-objects)
The KeyClass must omplement Equals, GetHashCode and be serializable.
The bags should specify <key> <column .../> <column ... in the same order.
Tables that have foreign keys/ many-to-one should also specify both columns.

After the id, add a <version name="Version" column="VERSION"> (Version should be an int; it's not a database TIMESTAMP).
Alternatively use <class ... optimistic-lock="all" to update ..where col=origValue. Or, with care, optimistic-lock="dirty"/dynamic-update="true" or select-before-update="true".

Property

• The default access of "property" is fine usually, but "nosetter.camelcase-underscore" (set via the private field) is useful for primary keys and foreign key collections, and when you have business logic/validation in setters. To not use the getter, use "field.camelcase-underscore" (if you use "field" without a naming strategy, you have to map the _camelCaseNames).
• Escape column and table names with backtick.
• Add insert="false" updates="false" for immutable columns. On class, use dynamic-update="true" to just update changed columns (with optimistic-lock="dirty" for optimistic locks).
• Properties can be wrapped with <component name="Address" class="Address"> which map those properties to an aggregated class/ value type (it has no identity and cannot be persisted- the common example is an Address class consisting of lines of address).
• There are some special types:
  Enums: specify the enum type or don't specify (reflection grabs the underlying enum type -eg Int32). Saves the underlying type.
  Booleans: type="YesNo" maps "Y" and "N" strings in the database to boolean; TrueFalse is the same for T and F
  For advanced stuff implement IUserType.

Collections- Foreign Keys

• Most foreign key relations can be mapped with <many-to-one> on one end and <bag>/<one-to-many> on the other marked with inverse="true". For large collections (100s of records or more- Product.Orders) don't do this.
  

  .Net collection	Mapping
  IEnumerable/ICollection/IList	<bag name="Children"> <key column="fkId"/> <one-to-many class="FKTable"/> </bag>
  IList with order	<list name="Children"> <key column="fkId"/> <list-index column="Order"/> <one-to-many class="FKTable"/> </list>
  Iesi.Collections.Iset (no duplicates) NB: no .Net 4 ISet, NH 3 is .Net 3.5 only	<set>
  IDictionary	<map>
  (Association table)	<idBag> with it's own id <collection-id column="AssocId" type="int"><generator class="guid-comb"/>...

• Set the default namespace and asssmbly in the hibernate-mapping root, otherwise you'll have to specify all joined objects as "Namespace.ClassName, Assembly"
• If you use soft-deletes (DELETEFLAG=1), a "where" will filter those out. For ordered collections (orders by order date) consider "order-by"
• <many-to-one not-found="ignore" allows invalid keys (inconsistent databases). BUT NH automatically loads the related entity immediately (so you get an extra SQL statement). You get the same issue if you use "property-ref" (referencing a property rather than the primary key).

Simple association tables can be mapped as many-to-many (no class is needed for the association table).