In this recipe, we'll show you how to generate all the necessary tables, columns, keys and relationships in your database - with two lines of code.
Complete the Configuring NHibernate with App.config recipe at the beginning of this chapter.
- Open
Program.cs. - Add these using statements to the beginning of the file:
using Eg.Core; using NHibernate.Mapping.ByCode; using NHibernate.Tool.hbm2ddl;
- Modify the
Mainmethod to look like this:var nhConfig = new Configuration().Configure(); var mapper=new ConventionModelMapper(); nhConfig.AddMapping(mapper.CompileMappingFor(new[] {typeof (TestClass)})); var schemaExport = new SchemaExport(nhConfig); schemaExport.Create(false, true); Console.WriteLine("The tables have been created")); Console.ReadKey(); - Build and run your application.
- Open your database and examine the tables. If everything worked, a table representing
TestClassshould have been created.
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. In order to demonstrate the functionality, we added a mapping for TestClass. How mappings are created and used will be further discussed in Chapter 2, Models and Mapping.
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 the IDataBaseSchema interface, such as the SQL Server, Oracle, MySQL, PostgreSQL, SQLite, and Firebird dialects.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. Similar toupdate, 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.