nHibernate save object in one-to-one relationship

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NHibernate

Hi! I'm a new in nHibernate help me to save an object and his connections in the base.
I've got the base with two tables: 1)person (idPerson, firstName, secondName, idService 2)service (idService, name)
The table service has 3 positions (gold, silver, brilliant)
I use the next method for saving:

public void Saves(Person entity)
    {
        using (var session = hibernateHelp.OpenSession())
        {
            using (var transaction = session.BeginTransaction())
            {
                ServiceRepository srp = new ServiceRepository();
                NHibernateUtil.Initialize(entity.service);
                session.Save(entity);
                transaction.Commit();
            }

        }
    }

The mapping is working and saving the new object in the table person and the table service when I send:

Person df=new Person{
           firstName="My",
           lastName="Go",
           status=true,
           serviceType=new ServiceType{serviceName="gold"}
           };

I need't to create the new note in the table service since it containts one. How to make the saver's method in order to save the link to the table server, but not create the new one???
I appreciate any links and suggestions.