Roundtrip engineering with NHibernate is possible using a set of commandline tools maintained as part of the NHibernate project, along with NHibernate support built into various code generation tools (MyGeneration, CodeSmith, ObjectMapper, AndroMDA).
The NHibernate main package comes bundled with the most important tool (it can even be used from "inside" NHibernate on-the-fly):
DDL schema generation from a mapping file (aka SchemaExport, hbm2ddl)
Other tools directly provided by the NHibernate project are delivered with a separate package, NHibernateContrib. This package includes tools for the following tasks:
mapping file generation from .NET classes marked with attributes (NHibernate.Mapping.Attributes, or NHMA for short)
Third party tools with NHibernate support are:
CodeSmith, MyGeneration, and ObjectMapper (mapping file generation from an existing database schema)
AndroMDA (MDA (Model-Driven Architecture) approach generating code for persistent classes from UML diagrams and their XML/XMI representation)
These 3rd party tools are not documented in this reference. Please refer to the NHibernate website for up-to-date information.
The generated schema includes referential integrity constraints (primary and foreign keys) for entity and collection tables. Tables and sequences are also created for mapped identifier generators.
You must specify a SQL Dialect via the hibernate.dialect property when using this tool.
Many NHibernate mapping elements define an optional attribute named length. You may set the length of a column with this attribute. (Or, for numeric/decimal data types, the precision.)
Some tags also accept a not-null attribute (for generating a NOT NULL constraint on table columns) and a unique attribute (for generating UNIQUE constraint on table columns).
Some tags accept an index attribute for specifying the name of an index for that column. A unique-key attribute can be used to group columns in a single unit key constraint. Currently, the specified value of the unique-key attribute is not used to name the constraint, only to group the columns in the mapping file.
Examples:
<property name="Foo" type="String" length="64" not-null="true"/> <many-to-one name="Bar" foreign-key="fk_foo_bar" not-null="true"/> <element column="serial_number" type="Int64" not-null="true" unique="true"/>
Alternatively, these elements also accept a child <column> element. This is particularly useful for multi-column types:
<property name="Foo" type="String">
<column name="foo" length="64" not-null="true" sql-type="text"/>
</property>
<property name="Bar" type="My.CustomTypes.MultiColumnType, My.CustomTypes"/>
<column name="fee" not-null="true" index="bar_idx"/>
<column name="fi" not-null="true" index="bar_idx"/>
<column name="fo" not-null="true" index="bar_idx"/>
</property>The sql-type attribute allows the user to override the default mapping of NHibernate type to SQL data type.
The check attribute allows you to specify a check constraint.
<property name="Foo" type="Int32">
<column name="foo" check="foo > 10"/>
</property>
<class name="Foo" table="foos" check="bar < 100.0">
...
<property name="Bar" type="Single"/>
</class>Table 21.1. Summary
| Attribute | Values | Interpretation |
|---|---|---|
| length | number | column length/decimal precision |
| not-null | true|false | specifies that the column should be non-nullable |
| unique | true|false | specifies that the column should have a unique constraint |
| index | index_name | specifies the name of a (multi-column) index |
| unique-key | unique_key_name | specifies the name of a multi-column unique constraint |
| foreign-key | foreign_key_name | specifies the name of the foreign key constraint generated for an association, use it on <one-to-one>, <many-to-one>, <key>, and <many-to-many> mapping elements. Note that inverse="true" sides will not be considered by SchemaExport. |
| sql-type | column_type | overrides the default column type (attribute of <column> element only) |
| check | SQL expression | create an SQL check constraint on either column or table |