Cascading delete on a Entity Framework Child that requires Either Parents but not both - entity-framework

Question

I have two classes that have a one to Many relationship to the child. The child can have only one parent, never both.
How can I make this possible? and if so how can I make it cascade delete if either parent get deleted?
class Parent1
{
public int Id {get;set;}
public IList<Child> Children {get;set;}
}
class Parent2
{
public int Id {get;set;}
public IList<Child> Children {get;set;}
}
class Child
{
public int Id {get;set;}
public int? Parent1Id {get;set;}
public int? Parent2Id {get;set;}
}

Answer 1

You can set a Entity Validation in EF to check this, but you also need to create a CHECK CONSTRAINT in your database to ensure that the rule is actually enforced.
Also you must have both FK properties and Navigation Properties on the Child entity to enforce this through EF. On a new Child the Navigation properties could be non-null, but the FK properties haven't been populated yet. On an existing entity, the FK properties will be loaded, but the Navigation Properties might not be. EG
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Data.Entity.Validation;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ef62test
{
class Program
{
public class Parent1
{
public int Id { get; set; }
public virtual ICollection<Child> Children { get; } = new HashSet<Child>();
}
public class Parent2
{
public int Id { get; set; }
public virtual ICollection<Child> Children { get; } = new HashSet<Child>();
}
public class Child
{
public int Id { get; set; }
public int? Parent1Id { get; set; }
public virtual Parent1 Parent1 { get; set; }
public int? Parent2Id { get; set; }
public virtual Parent2 Parent2 { get; set; }
}
public class MyDbContext : DbContext
{
public DbSet<Parent1> Parent1s { get; set; }
public DbSet<Parent2> Parent2s { get; set; }
public DbSet<Child> Children { get; set; }
protected override DbEntityValidationResult ValidateEntity(DbEntityEntry entityEntry, IDictionary<object, object> items)
{
var errors = new List<DbValidationError>();
if (entityEntry.Entity is Child child )
{
if ((child.Parent1Id != null || child.Parent1 != null)
&& (child.Parent2Id != null || child.Parent2 != null))
{
var error = new DbValidationError("Parent2id", "Parent2id must be null when Parent1id is null");
errors.Add(error);
}
}
return new DbEntityValidationResult(entityEntry, errors);
}
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
}
}
static void Main(string[] args)
{
Database.SetInitializer(new DropCreateDatabaseAlways<MyDbContext>());
using (var db = new MyDbContext())
{
db.Database.Log = s => Console.WriteLine(s);
var p1 = new Parent1();
var p2 = new Parent2();
var c = new Child();
p1.Children.Add(c);
p2.Children.Add(c);
db.Parent1s.Add(p1);
db.Parent2s.Add(p2);
db.Children.Add(c);
db.SaveChanges();
}
Console.WriteLine("Hit any key to exit.");
Console.ReadKey();
}
}
}