NHibernateBuyerPurchase.cs

public class Buyer
{
    public virtual int Id { get; set; }

    public virtual string Name { get; set; }
}

public class Purchase
{
    public virtual int Id { get; set; }

    public virtual decimal Sum { get; set; }

    public virtual int BuyerId { get; set; }

    public virtual Buyer Buyer { get; set; }
}

public class NHibernateHelper
{
    public static ISession OpenSession()
    {
        ISessionFactory sessionFactory = Fluently.Configure()
            .Database(MsSqlConfiguration.MsSql2012.ConnectionString(@"Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=tempdb;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=False;ApplicationIntent=ReadWrite;MultiSubnetFailover=False").ShowSql()
            )
            .Mappings(m =>
                m.FluentMappings
                    .AddFromAssemblyOf<Purchase>()
                    .AddFromAssemblyOf<Buyer>())
            .ExposeConfiguration(cfg =>
                new SchemaUpdate(cfg)
                    .Execute(false, true))
            .BuildSessionFactory();
        return sessionFactory.OpenSession();
    }
}

public class PurchaseMap : ClassMap<Purchase>
{
    public PurchaseMap()
    {
        Id(p => p.Id);
        Map(p => p.BuyerId);
        Map(p => p.Sum);
        References(p => p.Buyer);
    }
}

public class BuyerMap : ClassMap<Buyer>
{
    public BuyerMap()
    {
        Id(b => b.Id);
        Map(b => b.Name);
    }
}