Improve exception message for missing dependent in table splitting

[IndigoHealth]

Weird problem referencing owned types from classes that inherit from other classes. I created a simple repro. Please see https://stackoverflow.com/q/54246707/1637105

[ajcvickers]

Note for triage: simple repro:

public class IdentityUser
{
    public string Id { get; set; }
}

public class ApplicationUser : IdentityUser
{
    public PersonalName Name { get; set; }
    public ValidationToken ValidationToken { get; set; }
}

[Owned]
public class PersonalName
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

[Owned]
public class ValidationToken
{
    public int ValidationCode { get; set; }
    public DateTime ExperiationDateUTC { get; set; }
}

public class BloggingContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder
            .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Test;ConnectRetryCount=0");
    }

    public DbSet<IdentityUser> Users { get; set; }
    public DbSet<ApplicationUser> AppUsers { get; set; }
}

public class Program
{
    public static void Main()
    {
        using (var context = new BloggingContext())
        {
            context.Database.EnsureDeleted();
            context.Database.EnsureCreated();
        }
    }
}

Stack trace:

Unhandled Exception: System.InvalidOperationException: Cannot use table 'IdentityUser' for entity type 'ValidationToken' since it is being used for entity type 'PersonalName' and there is no relationship between their primary keys.
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidator.ValidateSharedTableCompatibility(IReadOnlyList`1 mappedTypes, String tableName)
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidator.ValidateSharedTableCompatibility(IModel model)
   at Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidator.Validate(IModel model)
   at Microsoft.EntityFrameworkCore.Internal.SqlServerModelValidator.Validate(IModel model)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ValidatingConvention.Apply(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.ImmediateConventionScope.OnModelBuilt(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.ModelBuilder.FinalizeModel()
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)
   at System.Lazy`1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor)
   at System.Lazy`1.CreateValue()
   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.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.Infrastructure.DatabaseFacade.get_DatabaseCreator()
   at Microsoft.EntityFrameworkCore.Infrastructure.DatabaseFacade.EnsureDeleted()
   at Program.Main() in C:\Stuff\TwoTwoPreview\TwoTwoPreview\Program.cs:line 82

[IndigoHealth]

Note the workaround in the answer to the original SO post. The problem only happens when the relationship is configured via attribute markup.

[IndigoHealth]

I thought that the workaround was to configure the owned relations in OnModelCreating, but I've added (and configured) a bunch of "owned" types to one of my inherited classes and now I get a new variant on the error:

[InvalidOperationException] The entity of type 'Patient' is sharing the table 'AspNetUsers' with entities of type 'EmergencyContact', but there is no entity of this type with the same key value that has been marked as 'Added'. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the key values.

Note that Patient derives from AspNetUsers, and EmergencyContact is a POCO class that doesn't contain an Id property.

[IndigoHealth]

Never mind... This is the error you get when you neglect to initialize the property that is a reference to the owned entity prior to adding the entity to the DB.

The error message is flat-out wrong. The error message should tell me that Patient.EmergencyContact can not be null. Or, if that level of detail isn't available, it should at least give me a clue that I need to initialize "owned" entity properties.

[AndriySvyryd]

Fixed in 6351aa1