public class Contact
    {
        private int id;
        public int Id
        {
            get { return id; }
        }
        public Company Company { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

    public class Company
    {
        private int id;
        public int Id
        {
            get { return id; }
        }

        public virtual string Name { get; set; }
        private IList<Contact> contacts = new List<Contact>();
        public virtual IList<Contact> Contacts
        {
            get { return contacts; }
            set { contacts = value; }
        }

        public void Add(Contact contact)
        {
            if (Contacts.Contains(contact) != false)
                return;
            Contacts.Add(contact);
            contact.Company = this;
        }
        public void Remove(Contact contact)
        {
            if (!Contacts.Contains(contact))
                return;
            Contacts.Remove(contact);
            contact.Company = null;
        }

    }

Company company = new Company {Name = "TEST"};
Contact contact = new Contact{FirstName = "Tommy", LastName = "Tester"};
company.Add(contact);
Repository.Save(company);
//Log would be:
//Company (13) created
//Contact (9) created

company.Name = "Test Co.";
Repository.Save(company);
//Log would be:
//Company (13) Name changed from "TEST" to "Test Co."

company.Remove(contact);
company.Add(new Contact{FirstName = "Blah", LastName = "Blah"});
Repository.Save(company);
//Log would be:
//Company (13) removed Contact (9)
//Company (13) added Contact (10)