Orphans are deleted when overlapping composite alternate key is mutated

Given the following simple Parent/Child model, where Child is an aggregate part of Parent (and cannot exist without it).

internal class Parent
{
    public Guid Id { get; private set; } = Guid.NewGuid();
    public virtual List<Child> Children { get; private set; } = new();
}

internal class Child
{
    public Guid Id { get; private set; } = Guid.NewGuid();
    public Guid ParentId { get; set; }
    public virtual Parent Parent { get; set; } = null!;
}

I want to prohibit a Parent from being deleted if it has Children, and I also want any Childthat is removed fromParent.Children` to be automatically deleted on save.

internal class ApplicationDbContext : DbContext
{
    public DbSet<Parent> Parents { get; set; } = null!;
    private DbSet<Child> Children { get; set; } = null!;

    public ApplicationDbContext(DbContextOptions options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Child>()
            .HasOne(x => x.Parent)
            .WithMany(x => x.Children)
            .OnDelete(DeleteBehavior.Restrict);
    }
}

As per the documentation, this code throws an exception because Child.Parent cannot be null.

var options = new DbContextOptionsBuilder<ApplicationDbContext>()
    .UseInMemoryDatabase("Test")
    .Options;

using (var context1 = new ApplicationDbContext(options))
{
    var parent = new Parent();
    parent.Children.Add(new Child());

    context1.Parents.Add(parent);
    context1.SaveChanges();
}

using (var context2 = new ApplicationDbContext(options))
{
    var parent = context2.Parents.Include(x => x.Children).First();
    parent.Children.RemoveAt(0);
    context2.SaveChanges();
}

It seems that modelBuilder.Entity<Child>().HasAlternateKey(x => new { x.Id, x.ParentId }) has the behaviour I expected, but @ajcvickers said “I certainly wouldn’t do it that way” - https://twitter.com/ajcvickers/status/1565633141880102914

What is the recommended practice to ensure the child is deleted instead?

[mrpmorris]

Thanks. To vote, do I just thumb it up?

[mrpmorris]

I really hope the priority of that report goes up. Otherwise it’s a lot of additional work to use EF for a DDD approach.