Configure Delete Behaviour in .NET Using EF Core Fluent API

public class Book
{
   public int Id { get; set; }
   public string Title { get; set; }
   // other properties as necessary
   public int AuthorId { get; set; }
   public Person Author { get; set; }
}
public class Person
{
   public int Id { get; set; }
   public string Name { get; set; }
   // Optionally add a navigation property to an authors books
   // public IList<Book> Books { get; } = [];
}

The above is an example of a required relationship by convention because the AuthorId is non-nullable. This means that when an author is deleted, all of the books they authored are also deleted.

To avoid deleting books when the author is deleted, make the AuthorId foreign-key property nullable like so

public int? AuthorId { get; set; }

And then configure the relationship in the OnModelCreating method

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
   base.OnModelCreating(modelBuilder);
   modelBuilder.Entity<Book>()
       .HasOne(u => u.Author)
       .WithMany()
       .HasForeignKey(u => u.AuthorId)
       .OnDelete(DeleteBehavior.SetNull);
}

This will set the foreign-key property of the dependent entities (i.e. the books) to null instead of deleting them. These entities are known as orphans.