Many-to-One relationship with primary key must be required

[ice1e0]

I want to set up an N-Hierarchy entity as follows:

public class MyEntry
{
	internal static void OnModelCreating(ModelBuilder modelBuilder)
	{
		modelBuilder.Entity<MyEntry>(entry =>
		{
			entry.ToTable("MyEntry");

			entry.HasKey(a => new { a.CompanyNum, a.Num })
				.HasName("PK_MyEntry");

			entry.HasOne(t => t.Company)
				.WithMany()
				.HasForeignKey(t => t.CompanyNum)
				.HasConstraintName("FK_MyEntry_Company_Num")
				.OnDelete(DeleteBehavior.Restrict);

			entry.HasOne(t => t.Parent)
				.WithMany()
				.HasForeignKey(t => new { t.CompanyNum, t.ParentNum })
				.HasConstraintName("FK_MyEntry_Parent_Num")
				.OnDelete(DeleteBehavior.Restrict);

			entry.HasMany(t => t.Children)
				.WithOne()
				.HasForeignKey(t => new {t.CompanyNum, t.Num})
				.HasPrincipalKey(t => new {t.CompanyNum, t.ParentNum})
				.IsRequired(false);
		});
	}

	[Required(AllowEmptyStrings = false)]
	[MaxLength(10)]
	public string CompanyNum { get; set; }

	public virtual Company Company { get; set; }

	[Required(AllowEmptyStrings = false)]
	[MaxLength(10)]
	public string Num { get; set; }

	[Required]
	[MaxLength(80)]
	public string Description { get; set; }

	[MaxLength(10)]
	public string ParentNum { get; set; }

	public virtual MyEntry Parent { get; set; }

	public virtual List<MyEntry> Children { get; set; }
}

I want to have the field ParentNum nullable because it is not required; at the same time I want to use the Children list field via the foreign key.
The problem is that ef core requires all fields from the foreign key being NOT NULL (or [Required]). When I want to enforce this by using the fluent API with .IsRequired(false) I get an exception (see below).

My expected behavior of the fluent API would be that if I use .isRequired(false) on a relationship, all fields are "optional required" (meaning: they are not required but can be made required by other conditions e.g. being a primary key or being defined with the attribute [Required]).

An alternative solution would be to add support to set a condition to the foreign key (similar to #10665).
Then I could just set a condition that the foreign key only applies when the ParentNum is not NULL.

Exception message: System.InvalidOperationException: The property 'CompanyNum' on entity type 'MyEntry' cannot be marked as nullable/optional because it has been included in a key {'CompanyNum', 'Num'}.
Stack trace:
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalPropertyBuilder.IsRequired(Boolean isRequired, ConfigurationSource configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalRelationshipBuilder.IsRequired(Boolean isRequired, ConfigurationSource configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Builders.ReferenceCollectionBuilder`2.IsRequired(Boolean required)
   at DuckAccounting.DataModel.Entites.MyEntry.<>c.<OnModelCreating>b__0_0(EntityTypeBuilder`1 entry)
   at Microsoft.EntityFrameworkCore.ModelBuilder.Entity[TEntity](Action`1 buildAction)
   at DuckAccounting.DataModel.Entites.MyEntry.OnModelCreating(ModelBuilder modelBuilder)
   at DuckAccounting.DataModel.DuckAccountingContext.OnModelCreating(ModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelCustomizer.Customize(ModelBuilder modelBuilder, DbContext context)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator)
   at System.Lazy`1.CreateValue()
   at System.Lazy`1.LazyInitValue()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies()
   at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.Internal.InternalAccessorExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

Further technical details

EF Core version: 2.1.2
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 8.1
IDE: Visual Studio 2017 15.6.6

[ajcvickers]

@ice1e0 Currently alternate keys have the same requirements as primary keys in that they cannot be nullable. Issue #4415 is tracking relaxing that requirement, so this is essentially a duplicate of that issue.