How to update database table schemas with NHibernate schema generation?

Question

I'm trying to figure out how to use NHibernate configuration with mapping to update table schemas, rather than dropping and recreating them.

Currently I'm using the NHibernate.Tool.hbm2ddl.SchemaExport obj with FluentNHibernate to generate the database schema for a mysql database. While I can't say it's a huge problem, whenever I call SchemaExport.Execute on the database, it's going to drop all the tables and then recreate them.

What would be way cooler is if I could just have it update the existing table structures retaining data where possible. But I don't really want to use a commerical product, or a code generator, because I don't like code generation in general, and I don't need this enough that I would consider paying for it. So hopefully any answer would keep these caveats in mind.

Answer 1

Check out SchemaUpdate.

Answer 2

The SchemaUpdate object provides database schema updating, by apparently generating and executing a series of SQL UPDATE statements (as well as constraint statements) when it's void Execute(bool script, bool doUpdate) function is called. The SchemaUpdate class is in the NHibernate.Tool.hbm2ddl namespace, which can be found in the Nhibernate.dll file.

SchemaUpdate is mentioned in chapter 15 of the nhibernate 1.0.2 toolset guide, here (section 15.1.5).

It's usage is described in "The NHibernate FAQ" which has a more complete example of how to use SchemaUpdate here. On this page the following example test is used, and there is a brief explanation of it's use.

[Test]
public void Update_an_existing_database_schema()
{
    _cfg = new Configuration();
    _cfg.Configure();
    _cfg.AddAssembly(Assembly.LoadFrom("DataLayer.dll"));
    var update = new SchemaUpdate(_cfg);
    update.Execute(true, false);
}

Here's a blog post I've written that has a little more detail about how SchemaUpdate works.

Answer 3

I'm using Fluent NH for mapping and NH as ORM in my C# application.. The problem is with schemaupdate class.. if I create an instance of that.. using the configuration I got from my exposeconfiguration function.. It doesnot update the schema.. I need some better documentation on SchemaUpdate class.. or might be.. setting the configuration to accomodate the new schema in an old database.. for versioning purposes..

I have tried a lot of variations but no DDL command is reflected on the actual database using SchemaUpdate :(