Adding multiple entities to a join table with a composite key throws

[pariesz]

Note: The exception doesn't occur if both if both properties in the composite key are different.

Exception message:
System.ArgumentNullException : Value cannot be null.
Parameter name: key

Stack trace:
   at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Collections.Generic.Dictionary`2.Remove(TKey key)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap`1.Remove(TKey key, InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IdentityMap`1.Remove(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.StopTracking(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges, Boolean forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.ConditionallyNullForeignKeyProperties(InternalEntityEntry dependentEntry, InternalEntityEntry principalEntry, IForeignKey foreignKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.ToDependentFixup(InternalEntityEntry dependentEntry, InternalEntityEntry principalEntry, IForeignKey foreignKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(InternalEntityEntry entry, ISet`1 handledForeignKeys, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges, Boolean forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(EntityEntryGraphNode node)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph(EntityEntryGraphNode node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(InternalEntityEntry rootEntry, EntityState entityState, Boolean forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState(InternalEntityEntry entry, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState[TEntity](TEntity entity, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.Add[TEntity](TEntity entity)
   at Microsoft.EntityFrameworkCore.Internal.InternalDbSet`1.Add(TEntity entity)
   at Tests.EntityFrameworkBug.Composite_Key_bug() in C:\Sites\CentralApi\Services\Core.Tests\EntityFrameworkBug.cs:line 19

Steps to reproduce

using Microsoft.EntityFrameworkCore;
using Xunit;

namespace Tests {
    public class EntityFrameworkBug {
        [Fact]
        public void Composite_Key_bug() {
            var opts = new DbContextOptionsBuilder<TestContext>().UseInMemoryDatabase("test").Options;
            using(var db = new TestContext(opts)) {
                var a = new AEntity();
                db.AEntities.Add(a);

                var b1 = new BEntity();
                var b2 = new BEntity();
                db.BEntities.AddRange(b1, b2);
                db.SaveChanges();

                db.EntityJoins.Add(new JoinEntity { EntityA = a, EntityB = b1 });
                db.EntityJoins.Add(new JoinEntity { EntityA = a, EntityB = b2 });
            }
        }

        public class AEntity {
            public int Id { get; set; }
        }
        public class BEntity {
            public int Id { get; set; }
        }
        public class JoinEntity {
            public int EntityAId { get; set; }
            public int EntityBId { get; set; }
            public virtual AEntity EntityA { get; set; }
            public virtual BEntity EntityB { get; set; }
        }
        public class TestContext : DbContext {
            public DbSet<AEntity> AEntities { get; set; }
            public DbSet<BEntity> BEntities { get; set; }
            public DbSet<JoinEntity> EntityJoins { get; set; }

            public TestContext(DbContextOptions<TestContext> options) : base(options) { }

            protected override void OnModelCreating(ModelBuilder builder) {
                builder
                    .Entity<AEntity>(c => { c.HasKey(x => x.Id); })
                    .Entity<BEntity>(c => { c.HasKey(x => x.Id); })
                    .Entity<JoinEntity>(c => {
                        c.HasKey(x => new { x.EntityAId, x.EntityBId });
                        c.HasOne(x => x.EntityA).WithOne().HasForeignKey<JoinEntity>(x => x.EntityAId);
                        c.HasOne(x => x.EntityB).WithOne().HasForeignKey<JoinEntity>(x => x.EntityBId);
                    });
            }
        }
    }
}

Further technical details

EF Core version: 2.0.0
Database Provider: In Memory
Operating system: Windows 10 Pro
IDE: Visual Studio Community 2017 Version 15.3.5

[ajcvickers]

@pariesz This is not a normal mapping for a "join table". It is instead two 1:1 relationships:

• AEntity (principal) maps to JoinEntity (dependent) using unique FK EntityAId
• BEntity (principal) maps to JoinEntity (dependent) using unique FK EntityBId

This means that EntityBId is being used as a unique FK on it's own--that is, not part of the composite--such that adding to entities with the same EntityBId will fail. It shouldn't throw the exception you are seeing--that is a bug, but it should fail at least when saving to the database.

Did you intend to setup a join table like that used in a many-to-many relationship? If so, then try this mapping:

c.HasOne(x => x.EntityA).WithMany().HasForeignKey(x => x.EntityAId);
c.HasOne(x => x.EntityB).WithMany().HasForeignKey(x => x.EntityBId);

[pariesz]

Thanks Arthur for looking at that so quickly!

I had configured the relationships badly, using WithMany() fixed it. I haven't closed the issue as you said you want to work on a clearer exception.

[ajcvickers]

No longer throws bad exception in preview2 bits.