DatabaseDeleter.cs

public class DatabaseDeleter
{
    private readonly ISessionFactory _configuration;
    private static readonly string[] _ignoredTables = new[] { "sysdiagrams", /* DB Migration tables too */ };
    private static string[] _tablesToDelete;
    private static string _deleteSql;
    private static object _lockObj = new object();
    private static bool _initialized;

    public DatabaseDeleter(ISessionFactory sessionSource)
    {
        _configuration = sessionSource;

        BuildDeleteTables();
    }

    private class Relationship
    {
        public string PrimaryKeyTable { get; private set; }
        public string ForeignKeyTable { get; private set; }
    }

    public virtual void DeleteAllData()
    {
        ISession session = _configuration.OpenSession();

        using (IDbCommand command = session.Connection.CreateCommand())
        {
            command.CommandText = _deleteSql;
            command.ExecuteNonQuery();
        }
    }

    public static string[] GetTables()
    {
        return _tablesToDelete;
    }

    private void BuildDeleteTables()
    {
        if (!_initialized)
        {
            lock (_lockObj)
            {
                if (!_initialized)
                {
                    ISession session = _configuration.OpenSession();

                    var allTables = GetAllTables(session);

                    var allRelationships = GetRelationships(session);

                    _tablesToDelete = BuildTableList(allTables, allRelationships);

                    _deleteSql = BuildTableSql(_tablesToDelete);

                    _initialized = true;
                }
            }
        }
    }

    private static string BuildTableSql(IEnumerable<string> tablesToDelete)
    {
        string completeQuery = "";
        foreach (var tableName in tablesToDelete)
        {
            completeQuery += String.Format("delete from [{0}];", tableName);
        }
        return completeQuery;
    }

    private static string[] BuildTableList(ICollection<string> allTables, ICollection<Relationship> allRelationships)
    {
        var tablesToDelete = new List<string>();

        while (allTables.Any())
        {
            var leafTables = allTables.Except(allRelationships.Select(rel => rel.PrimaryKeyTable)).ToArray();

            tablesToDelete.AddRange(leafTables);

            foreach (var leafTable in leafTables)
            {
                allTables.Remove(leafTable);
                var relToRemove = allRelationships.Where(rel => rel.ForeignKeyTable == leafTable).ToArray();
                foreach (var rel in relToRemove)
                {
                    allRelationships.Remove(rel);
                }
            }
        }

        return tablesToDelete.ToArray();
    }

    private static IList<Relationship> GetRelationships(ISession session)
    {
        var otherquery = session.CreateSQLQuery(@"select
  														so_pk.name as PrimaryKeyTable
														,   so_fk.name as ForeignKeyTable
														from
															sysforeignkeys sfk
																inner join sysobjects so_pk on sfk.rkeyid = so_pk.id
																inner join sysobjects so_fk on sfk.fkeyid = so_fk.id
														order by
															so_pk.name
														,   so_fk.name");

        return otherquery.SetResultTransformer(Transformers.AliasToBean<Relationship>()).List<Relationship>();
    }

    private static IList<string> GetAllTables(ISession session)
    {
        var query = session.CreateSQLQuery("select t.name from sys.tables t INNER JOIN sys.schemas s ON t.schema_id = s.schema_id WHERE s.name = 'dbo'");

        return query.List<string>().Except(_ignoredTables).ToList();
    }
}