DeleteBehavior is not updated to Cascade if requiredness is set on FK after the relationship is discovered

[divega]

Deletes on required associations are supposed to cascade unless the user explicitly opts out. It seems that if the requiredness of the association is set after we first learn about it the delete behavior doesn't get updated.

Repro:

using System;
using System.Data.SqlClient;
using System.Linq;
using Microsoft.Data.Entity;
using Microsoft.Data.Entity.Infrastructure;
using Microsoft.Data.Entity.Metadata;

namespace ConsoleApplication20
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            using (var context = new IdentityDbContext())
            {
                var fk =
                    context.Model.FindReferencingForeignKeys(context.Model.FindEntityType(typeof(Employee))).First();
                Console.WriteLine(
                    $"FK on Employee.Detail is required: {fk.IsRequired}");
                Console.WriteLine(
                    $"Delete behavior on Employee.Detail is {fk.DeleteBehavior}");  // Required
            }
        }
    }

    public class EmployeeDetail
    {
        public virtual string EmployeeId { get; set; }
        public virtual byte[] Photo { get; set; }
    }

    public class Employee
    {
        public virtual string Id { get; set; }
        public virtual EmployeeDetail Detail { get; set; }
    }

    public class IdentityDbContext : DbContext
    {
        public DbSet<Employee> Users { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Employee>(b =>
            {
                b.HasOne(u => u.Detail)
                    .WithOne()
                    .HasForeignKey<EmployeeDetail>(pd => pd.EmployeeId);
            });

            // if key configuration of EmployeeDetail happens before the association configuration 
            // delete behavior will correctly be set to cascade 
            modelBuilder.Entity<EmployeeDetail>(b =>
            {
                b.HasKey(r => r.EmployeeId);
            });
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseLocalDB(GetType().Name);
        }
    }

    public static class DbContextOptionsExtensions
    {
        public static SqlServerDbContextOptionsBuilder UseLocalDB(this DbContextOptionsBuilder optionsBuilder,
            string databaseName = null, Action<SqlConnectionStringBuilder> action = null)
        {
            var connectionStringBuilder = new SqlConnectionStringBuilder
            {
                DataSource = "(localdb)\\mssqllocaldb",
                InitialCatalog = databaseName
            };
            action?.Invoke(connectionStringBuilder);
            return optionsBuilder.UseSqlServer(connectionStringBuilder.ConnectionString);
        }
    }
}