NHibernate database versioning: object level schema and data upgrades - nhibernate

Question

I would like to approach database versioning and automated upgrades in NHibernate from a different direction than most of the strategies proposed out there.
As each object is defined by an XML mapping, I would like to take size and checksum for each mapping file/ configuration and store that in a document database (raven or something) along with a potential custom update script. If no script is found, use the NHibernate DDL generator to update the object schema. This way I can detect changes, and if I need to make DML changes in addition to DDL, or perform a carefully ordered transformation, I can theoretically do so in a controlled, testable manner. This should also maintain a certain level of persistence-layer agnosticism, although I'd imagine the scripts would still necessarily be database system-specific.
The trick would be, generating the "old" mapping files from the database and comparing them to the current mapping files. I don't know if this is possible. I also don't know if I'm missing anything else that would make this strategy prohibitively impractical.
My question, then: how practical is this strategy, and why?

Answer 1

what i did to solve just that problem
version the database in a table called SchemaVersion
query the table to see if schema is up to date (required version stored in DAL), if yes goto 6.
get updatescript with version == versionFromBb from resources/webservices/...
run the script which also alters the schemaversion to the new version
goto 2.
run app
to generate the scripts i have used 2 options
support one rdbms: run SchemaUpdate to export into file and add DML statements manually
support multiple rdbms: use Nhibernate class Table to generate at runtime ddl to add/alter/delete tables and code which uses a session DML
Update:
"what method did you use to store the current version"
small example
something like this
public static class Constants
{
public static readonly Version DatabaseSchemaVersion = new Version(1, 2, 3, 4);
}
public class DBMigration
{
private IDictionary<Version, Action> _updates = new Dictionary<Version, Action>();
private Configuration _config;
private Dialect _dialect;
private IList<Action<ISession>> _actions = new List<Action<ISession>>(16);
private string _defaultCatalog;
private string _defaultSchema;
private void CreateTable(string name, Action<Table> configuretable)
{
var table = new Table(name);
configuretable(table);
string createTable = table.SqlCreateString(_dialect, _config.BuildMapping(), _defaultCatalog, _defaultSchema);
_actions.Add(session => session.CreateSQLQuery(createTable).ExecuteUpdate());
}
private void UpdateVersionTo(Version version)
{
_actions.Add(session => { session.Get<SchemaVersion>(1).Value = version; session.Flush(); });
}
private void WithSession(Action<session> action)
{
_actions.Add(action);
}
public void Execute(Configuration config)
{
_actions.Clear();
_defaultCatalog = config.Properties[NH.Environment.DefaultCatalog];
_defaultSchema = config.Properties[NH.Environment.DefaultSchema];
_config = config;
_dialect = Dialect.GetDialect(config.Properties);
using (var sf = _config.BuildSessionFactory())
using (var session = sf.OpenSession())
using (var tx = session.BeginTransaction())
{
Version dbVersion = session.Get<SchemaVersion>(1).Value;
while (dbVersion < Constants.DatabaseSchemaVersion)
{
_actions.Clear();
_updates[dbVersion].Invoke(); // init migration, TODO: error handling
foreach (var action in _actions)
{
action.Invoke(session);
}
tx.Commit();
session.Clear();
dbVersion = session.Get<SchemaVersion>(1).Value;
}
}
}
public DBMigration()
{
_updates.Add(new Version(1, 0, 0, 0), UpdateFromVersion1);
_updates.Add(new Version(1, 0, 1, 0), UpdateFromVersion1);
...
}
private void UpdateFromVersion1()
{
AddTable("Users", table => table.AddColumn(...));
WithSession(session => session.CreateSqlQuery("INSERT INTO ..."));
UpdateVersionTo(new Version(1,0,1,0));
}
...
}