In Chapter 1, we built mappings to map our persistent classes to the database, but we haven't built that database.
In this recipe, I'll show you how to generate all the necessary tables, columns, keys and relationships in your database from your mappings—with two lines of code.
- Complete the Configuring NHibernate with App.config recipe at the beginning of this chapter.
- Install Microsoft SQL Server 2008 Express on your PC, using the default settings.
- Create a blank database named NHCookbook.
- Open
Program.cs. - Add the using statement:
usingNHibernate.Tool.hbm2ddl;.
- Add the following lines to the end of
Main.var schemaExport = new SchemaExport(nhConfig); schemaExport.Create(false, true);
- Build and run your application.
- Open your database and examine the tables.
The hbm2ddl (hibernate mapping to data definition language) tool uses the mapping metadata in the configuration object to build a SQL script of our database objects. It then connects to our database and runs this script.
Alternatively, we can use the hbm2ddl.auto configuration property to build our database schema automatically whenever our application calls BuildSessionFactory. We can set the property to the following values:
update: TheSchemaUpdateclass updates our database schema, avoiding destructive changes. This only works for dialects that implement theIDataBaseSchemainterface.create: TheSchemaExportclass creates our database schema from scratch for a fresh database.create-drop:SchemaExportrecreates the database schema by first dropping and then creating each table.validate: TheSchemaValidateclass compares the existing database schema to the schema NHibernate expects, based on your mappings. Likeupdate, this requires a dialect that implementsIDataBaseSchema.
While create-drop is immensely helpful during development, only validate is suggested for production environments, as the tiniest mistake can have huge consequences. Rather, you should script the database, as shown in the next recipe, and run the script explicitly to set up your production database.