How to force an Exception when trying to delete a Master record that already contains relationship in a Child Record?

Question

I created the following structure in SQL Server:

Using Entityframework with Code-First the classes stayed that way:

[Table("Master")]
public class Master
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int Id { get; set; }
}

[Table("Child")]
public class Child
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    public string Description { get; set; }

    [ForeignKey("Master")]
    public int? MasterId { get; set; }
    public virtual Master Master { get; set; }
}

See that MasterId for being null (int?).

Still, setting the Foreign key raised in Child to not delete in cascade (setando In Action) expected that when trying to delete a record on Master, which has already been related to Child, generate a Exception.

Mindful of the fact that I have removed the cascading deletion conventions:

public class DataContext : DbContext
{
    public DataContext()
        : base("Data Source=(local); Initial Catalog=TestFK; Integrated Security=True")
    {
        this.Configuration.LazyLoadingEnabled = false;
        this.Configuration.ProxyCreationEnabled = false;
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();
        modelBuilder.Conventions.Remove<ManyToManyCascadeDeleteConvention>();
        base.OnModelCreating(modelBuilder);
    }

    public DbSet<Master> Masters { get; set; }
    public DbSet<Child> Childs { get; set; }
}

So I tested this hoping to get one Exception, but it did not occur:

class Program
{
    static void Main(string[] args)
    {
        using (var ctx = new DataContext())
        {
            ctx.Masters.Add(new Master { Id = 22 });
            ctx.Childs.Add(new Child { Description = "Teste 1", MasterId = 22 });
            ctx.SaveChanges();

            var versao = ctx.Masters.SingleOrDefault(x => x.Id == 22);
            if (versao != null)
            {
                ctx.Masters.Remove(versao); // <-- esperava que isso geraria uma exception
                ctx.SaveChanges();
            }
        }
    }
}

Otherwise, the record was deleted and the value in Child was set to null.

How to force this Exception?
Missing any settings? It’s standard behavior?

---

To conclude, the operation being done directly in the database returns a Exception, as expected.

Answer 1

It’s a standard behavior?

Yeah. Notice your Master (the parent record) can be null on the child (it is called the "orphan record"), so there would be no reason to generate an exception since your bank allows this exclusion. What the Entity Framework does is tidy up the records for you to ensure relational consistency.

In this case, making an exception would be a business rule:

class Program
{
    static void Main(string[] args)
    {
        using (var ctx = new DataContext())
        {
            ctx.Masters.Add(new Master { Id = 22 });
            ctx.Childs.Add(new Child { Description = "Teste 1", MasterId = 22 });
            ctx.SaveChanges();

            var versao = ctx.Masters.SingleOrDefault(x => x.Id == 22);

            if (ctx.Childs.Any(c => c.MasterId == versao.Id)) {
                throw new Exception("Não é possível excluir registro pai que tenha filhos.");
            }

            // Comentei porque SingleOrDefault dá exceção quando o registro vem nulo, 
            // então esse condicional não faz muito sentido.
            // if (versao != null)
            // {
                ctx.Masters.Remove(versao); // <-- esperava que isso geraria uma exception
                ctx.SaveChanges();
            // }
        }
    }
}

About the exception

The Entity Framework first defines MasterId in Child as null before excluding Master because you know the relationship and you know there’s a foreign key there.