Improve exception when defining a relationship to a keyless entity type

[seattleite7]

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;
using System;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;

namespace EFCoreNullException
{
    class Summary
    {
        [Column("userId")]
        public long UserID { get; set; }
        [Column("hours")]
        public int Hours { get; set; }
        public ContactInfo Contact { get; set; } = new ContactInfo();
    }

    class ContactInfo
    {
        [Column("userId")]
        public long UserID { get; set; }
        [Column("name")]
        public string Name { get; set; } = "";
    }

    class MyContext : DbContext
    {
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseMySql("host=localhost;database=test;user id=test;password=test", x => x.ServerVersion("8.0.16-mysql"));
            }
        }
        protected override void OnModelCreating(ModelBuilder builder)
        {
            builder.Entity<ContactInfo>().HasNoKey();
            builder.Entity<Summary>().HasKey(e => e.UserID);
            builder.Entity<Summary>().HasOne(e => e.Contact).WithOne().HasPrincipalKey<Summary>(e => e.UserID).HasForeignKey<ContactInfo>(b => b.UserID);
            builder.Entity<Summary>().ToTable("summaries");
            builder.Entity<ContactInfo>().ToTable("contacts");
        }
        public DbSet<Summary> Summaries { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyContext context = new MyContext();
            IQueryable<Summary> query = context.Summaries.Where(e => e.UserID == 1);
            Summary t = query.FirstOrDefault();
            Console.WriteLine(t.Contact.Name);
        }
    }
}

System.NullReferenceException: 'Object reference not set to an instance of an object.'  
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.SetOrAddForeignKey(ForeignKey foreignKey, InternalEntityTypeBuilder principalEntityTypeBuilder, IReadOnlyList`1 dependentProperties, Key principalKey, String navigationToPrincipalName, Nullable`1 isRequired, Nullable`1 configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.CreateForeignKey(InternalEntityTypeBuilder principalEntityTypeBuilder, IReadOnlyList`1 dependentProperties, Key principalKey, String navigationToPrincipalName, Nullable`1 isRequired, ConfigurationSource configurationSource)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.HasRelationship(EntityType targetEntityType, Nullable`1 navigationToTarget, Nullable`1 inverseNavigation, Boolean setTargetAsPrincipal, ConfigurationSource configurationSource, Nullable`1 required)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalEntityTypeBuilder.HasRelationship(EntityType targetEntityType, MemberInfo navigationProperty, ConfigurationSource configurationSource, Boolean setTargetAsPrincipal)
   at Microsoft.EntityFrameworkCore.Metadata.Builders.EntityTypeBuilder`1.HasOne[TRelatedEntity](Expression`1 navigationExpression)
   at EFCoreNullException.MyContext.OnModelCreating(ModelBuilder builder) in C:\Users\Eric\source\repos\EFCoreNullException\Program.cs:line 65
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelCustomizer.Customize(ModelBuilder modelBuilder, DbContext context)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__7_3(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   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.DbContext.get_DbContextDependencies()
   at Microsoft.EntityFrameworkCore.DbContext.get_Model()
   at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.get_EntityType()
   at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.CheckState()
   at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.get_EntityQueryable()
   at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.System.Linq.IQueryable.get_Provider()
   at System.Linq.Queryable.Where[TSource](IQueryable`1 source, Expression`1 predicate)
   at EFCoreNullException.Program.Main(String[] args) in \EFCoreNullException\Program.cs:line 81

Further technical details

EF Core version: Microsoft.EntityFrameworkCore 3.1.2
Database provider: Pomelo.EntityFrameworkCore.MySql 3.1.1
Target framework: .NET Core 3.1.0
Operating system: Windows 10
IDE: Microsoft Visual Studio Community 2019 Version 16.4.2

[ajcvickers]

@seattleite7 What is your intention by using builder.Entity<ContactInfo>().HasNoKey(); here?

[seattleite7]

@ajcvickers I was going off the error message "The type requires a primary key to be defined. If you intend to use a keyless type, call HasNoKey()". But now I get it has to have a primary key even if the foreign key is unique. Full disclosure, I am very new to EF.

[ajcvickers]

@seattleite7 Welcome to EF Core! :-)

There's a few ways that you could map these types. Taking the lead from your code, something like this will map the types as normal entity types:

protected override void OnModelCreating(ModelBuilder builder)
{
    builder.Entity<ContactInfo>().HasKey(e => e.UserID);
    builder.Entity<ContactInfo>().ToTable("contacts");

    builder.Entity<Summary>().HasKey(e => e.UserID);
    builder.Entity<Summary>()
        .HasOne(e => e.Contact)
        .WithOne()
        .HasPrincipalKey<Summary>(e => e.UserID)
        .HasForeignKey<ContactInfo>(b => b.UserID);
    
    builder.Entity<Summary>().ToTable("summaries");
}

When writing a query against this, you'll need to use Incldue to load the related ContactInfo:

var query = context.Summaries.Include(e => e.Contact).Where(e => e.UserID == 1);

Another option would be to use an owned type. This is more appropriate if the ContactInfo entity is conceptually part of the Summary aggregate. It means that:

• ContactInfo will be automatically included when loading a Summary
• ContactInfo doesn't need to declare it's own key property.

Note that in either case, this code is problematic:

public ContactInfo Contact { get; set; } = new ContactInfo();

This creates a new ContactInfo entity instance every time a Summary is created, which in turn leads to ambiguity if EF loads an entity instance. See #18007 for an in-depth discussion/

[AndriySvyryd]

Duplicate of #18149