/// <summary>
/// Default constructor assigning new <see cref="Guid"/> becuase entity live also un-persisted.
/// </summary>
public Rental(Book book, string rentee) {
AssertUtils.IsTrue(book != null);
AssertUtils.IsTrue(rentee != null);
Id = new Guid();
Book = book;
Rentee = rentee;
StartOfRental = DateTime.UtcNow;
}
public void AddBook(Book book) {
books.Add(book);
}