3.级联删除

using (var context = new BookStore())
{    
    var author = context.Authors
        .FirstOrDefault();
    
    context.Authors.Remove(author);
    context.SaveChanges();
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Book>()
        .HasOptional<Author>(b => b.Author)
        .WithMany()
        .WillCascadeOnDelete(false);
}