Cosmos: Allow PK with just the partition key and using id as the partition key

[Paul-Beliavskis]

When setting HasNoDiscriminator on the dbContext entity and the partitionkey and primary key are the same there is an error thrown when I try to add an entity to the context.

Steps to reproduce

My db context looks like this:

    public class UserDbContext : DbContext
    {
        public UserDbContext(DbContextOptions<UserDbContext> options)
            : base(options)
        {
        }

        public DbSet<User> Users { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.HasDefaultContainer("Users");
            modelBuilder.Entity<User>()
                        .HasPartitionKey(u => u.UserId)
                        .HasNoDiscriminator();
        }
    }

Notice the HasNoDiscriminator is added and the partition key is the primary key.

Then when I add to this DBContext like so:
var newAdmin = await _userDbContext.Users.AddAsync(admin, cancellationToken);

I get an exception with the following stack trace:

   at System.Text.StringBuilder.Remove(Int32 startIndex, Int32 length)
   at Microsoft.EntityFrameworkCore.Cosmos.ValueGeneration.Internal.IdValueGenerator.NextValue(EntityEntry entry)
   at Microsoft.EntityFrameworkCore.ValueGeneration.ValueGenerator.NextValueAsync(EntityEntry entry, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ValueGeneration.ValueGenerator.NextAsync(EntityEntry entry, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ValueGenerationManager.<GenerateAsync>d__8.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.<SetEntityStateAsync>d__24.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.<PaintActionAsync>d__5.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.<TraverseGraphAsync>d__1`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.DbContext.<AddAsync>d__71`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
   at Panviva.Services.Configuration.Application.Handlers.RequestHandlers.CommandHandlers.PostOrganizationHandler.<Handle>d__8.MoveNext() in C:\Panviva\io-panviva-services-configuration\Panviva.Services.Configuration\Panviva.Services.Configuration.Application\Handlers\RequestHandlers\CommandHandlers\PostOrganizationHandler.cs:line 125
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestPostProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestPreProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at MediatR.Pipeline.RequestExceptionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at MediatR.Pipeline.RequestExceptionActionProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestPostProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestPreProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MediatR.Pipeline.RequestPreProcessorBehavior`2.<Handle>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Panviva.Services.Configuration.Controllers.OrganizationsController.<PostOrganization>d__10.MoveNext() in C:\Panviva\io-panviva-services-configuration\Panviva.Services.Configuration\Panviva.Services.Configuration\Controllers\OrganizationsController.cs:line 207

After looking at the code in https://github.com/dotnet/efcore/blob/master/src/EFCore.Cosmos/ValueGeneration/Internal/IdValueGenerator.cs
I believe I have found the issue. I will create a pr for it.

Further technical details

EF Core version: 3.1.5
Database provider: (e.g. Microsoft.EntityFrameworkCore.CosmosDb)
Target framework: 3.1(e.g. .NET Core 3.0)
Operating system: Windows 10 Enterprise
IDE: Visual Studio 2019 16.6

[AndriySvyryd]

This isn't a supported scenario. If the only property in the primary key is the partition key then the id value will always be the same as the partition key value meaning there will be only one item per partition resulting in very bad perf. You'd be better off not configuring the partition key at all.

[Paul-Beliavskis]

This isn't a supported scenario. If the only property in the primary key is the partition key then the id value will always be the same as the partition key value meaning there will be only one item per partition resulting in very bad perf. You'd be better off not configuring the partition key at all.

But what if most of our transactions are simply looking up user objects by id. It was my understanding that cosmos would be able to go directly to that partition and get that object instead of searching through every partition if the id I am using to search for is the partition key. The documentation also states that the partition key should be as unique as possible:
https://docs.microsoft.com/en-us/azure/cosmos-db/partitioning-overview

Did I mis-interpret the doc?

Also in the case where you say it is not supported, why throw an exception that doesn't tell you anything about the fact that this scenario is not supported? The exception gets thrown line 69 in https://github.com/dotnet/efcore/blob/master/src/EFCore.Cosmos/ValueGeneration/Internal/IdValueGenerator.cs
When it tries to remove the pipe on the end of the string.

It's also quite interesting that when I set the partition key but do not set the HasNoDiscriminator then the id field has the value of the Discriminator which means many items end up having the same Id which doesn't make sense to me.

I propose either throwing an appropriate exception or just allowing the id field to contain the partition key.

[AndriySvyryd]

Did I mis-interpret the doc?

You read it correctly. It was updated since the last time I read it, we'll change our recommendation to match.