Combination of `GroupBy`, `FirstOrDefault` and `Select` throws a KeyNotFoundException

[GerardSmit]

If you use GroupBy, Select that gets an row from the group with FirstOrDefault and then try to reduce the amount of columns returned with another Select, you get an KeyNotFoundException:

var reports = await context.ReportItems
    .GroupBy(e => e.Report.Name)
    .Select(g => new
    {
        g.Key,
        // "g.MaxBy(x => x.Time)" is not supported by EF Core
        MaxTime = g.OrderByDescending(i => i.Time).FirstOrDefault()
    })
    .Where(i => i.MaxTime != null)
    .Select(i => new
    {
        Name = i.Key,
        TimeName = i.MaxTime.Name,
        i.MaxTime.Time
    })
    .ToListAsync();

Full code

.csproj

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net7.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

    <ItemGroup>
      <PackageReference Include="Microsoft.EntityFrameworkCore" Version="7.0.2" />
      <PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="7.0.2" />
      <PackageReference Include="System.Linq.Async" Version="6.0.1" />
    </ItemGroup>

</Project>

Program.cs

using Microsoft.Data.Sqlite;
using Microsoft.EntityFrameworkCore;

await using var connection = new SqliteConnection("DataSource=:memory:");

await connection.OpenAsync();

var options = new DbContextOptionsBuilder<AppDbContext>()
    .UseSqlite(connection)
    .Options;

await using var context = new AppDbContext(options);

await context.Database.EnsureCreatedAsync();

var newReport = new Report
{
    Name = "Report 1",
};

context.Reports.Add(newReport);

context.ReportItems.Add(new ReportItem
{
    Name = "Item 1",
    Report = newReport,
    Time = 10
});

context.ReportItems.Add(new ReportItem
{
    Name = "Item 2",
    Report = newReport,
    Time = 20
});

context.ReportItems.Add(new ReportItem
{
    Name = "Item 3",
    Report = newReport,
    Time = 30
});

await context.SaveChangesAsync();

var reports = await context.ReportItems
    .GroupBy(e => e.Report.Name)
    .Select(g => new
    {
        g.Key,
        // "g.MaxBy(x => x.Time)" is not supported by EF Core
        MaxTime = g.OrderByDescending(i => i.Time).FirstOrDefault()
    })
    .Where(i => i.MaxTime != null)
    .Select(i => new
    {
        Name = i.Key,
        TimeName = i.MaxTime.Name,
        i.MaxTime.Time
    })
    .ToListAsync();

foreach (var report in reports)
{
    Console.WriteLine($"{report.Name} - {report.TimeName} {report.Time}");
}

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

    public DbSet<Report> Reports { get; set; } = null!;

    public DbSet<ReportItem> ReportItems { get; set; } = null!;
}

public class Report
{
    public int Id { get; set; }

    public required string Name { get; set; }

    public List<ReportItem> Items { get; set; } = new();
}

public class ReportItem
{
    public int Id { get; set; }

    public required string Name { get; set; }

    public required Report Report { get; set; }

    public required int Time { get; set; }
}

Expected output

Report 1 - Item 3 30

Notes

Changing the query to First without the null-check also throws an exception:

var reports = await context.ReportItems
    .GroupBy(e => e.Report.Name)
    .Select(g => new
    {
        g.Key,
        MaxTime = g.OrderByDescending(i => i.Time).First()
    })
    .Select(i => new
    {
        Name = i.Key,
        TimeName = i.MaxTime.Name,
        i.MaxTime.Time
    })
    .ToListAsync();

Changing the query to start from Reports (without GroupBy) and then get the largest one, the query works as expected:

var reports = await context.Reports
    .Select(g => new
    {
        g.Name,
        MaxTime = g.Items.OrderByDescending(i => i.Time).FirstOrDefault()
    })
    .Where(i => i.MaxTime != null)
    .Select(i => new
    {
        i.Name,
        TimeName = i.MaxTime.Name,
        i.MaxTime.Time
    })
    .ToListAsync();

If I add an AsAsyncEnumerable before the select, the query works as intended:

var reports = await context.ReportItems
    .GroupBy(e => e.Report.Name)
    .Select(g => new
    {
        g.Key,
        MaxTime = g.OrderByDescending(i => i.Time).First()
    })
    .AsAsyncEnumerable()
    .Select(i => new
    {
        Name = i.Key,
        TimeName = i.MaxTime.Name,
        i.MaxTime.Time
    })
    .ToListAsync();

Stack traces

System.Collections.Generic.KeyNotFoundException: The given key 'EmptyProjectionMember' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.GetProjection(ProjectionBindingExpression projectionBindingExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.BindProperty(EntityReferenceExpression entityReferenceExpression, IProperty property)
   at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.TryBindMember(Expression source, MemberIdentity member)
   at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.VisitMember(MemberExpression memberExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.TranslateInternal(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.Translate(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitNew(NewExpression newExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass12_0`1.<ExecuteAsync>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetAsyncEnumerator(CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.ConfiguredCancelableAsyncEnumerable`1.GetAsyncEnumerator()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
   at Program.<Main>$(String[] args) in C:\Sources\IssueReport\IssueReport\Program.cs:line 46
   at Program.<Main>$(String[] args) in C:\Sources\IssueReport\IssueReport\Program.cs:line 63
   at Program.<Main>$(String[] args) in C:\Sources\IssueReport\IssueReport\Program.cs:line 63
   at Program.<Main>(String[] args)

Include provider and version information

EF Core version: 7.0.2 Database provider: bug found in Microsoft.EntityFrameworkCore.SqlServer reproduced in Microsoft.EntityFrameworkCore.Sqlite Target framework: .NET 7.0 (SDK: 7.0.200, 27f0a7fa5a) Operating system: Windows 11 (22H2) IDE: Rider 2022.3.1

[wqoq]

Hello. I'll just add that I get the same error doing pretty much the exact same thing. Slight difference is that I'm using SqlServer and LocalDB, targeting .NET 6, and using EF Core 7.0.5 with code-first migrations. I made a small runnable sample project that exhibits the error, too.

.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <RootNamespace>EF7_group_by_select</RootNamespace>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="7.0.5">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="7.0.5" />
  </ItemGroup>

  <ItemGroup>
    <Folder Include="Migrations\" />
  </ItemGroup>

</Project>

Program.cs

using Microsoft.EntityFrameworkCore;

namespace EF7_group_by_select
{
    public class Program
    {
        static async Task Main()
        {
            var dbContext = new MyContext();

            var filter = new[] { "foo", "bar" };

            var listOfIds = await dbContext.Items
                .Where(x => filter.Contains(x.RefId))
                .GroupBy(x => x.RefId)
                .Select(grp => grp.OrderByDescending(x => x.Date).First())
                .Select(x => x.Id)
                .ToListAsync();

            Console.WriteLine(string.Join(Environment.NewLine, listOfIds));
        }
    }

    public class MyContext : DbContext
    {
        public DbSet<Item> Items { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=example-db;Integrated Security=True");
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Item>().HasData(
                new Item { Id = "a1", Date = new(2023, 6, 12), RefId = "foo" },
                new Item { Id = "b2", Date = new(2023, 5, 15), RefId = "bar" },
                new Item { Id = "c3", Date = new(2023, 1, 10), RefId = "baz" },
                new Item { Id = "d4", Date = new(2023, 2, 25), RefId = "foo" },
                new Item { Id = "e5", Date = new(2023, 3, 18), RefId = "bar" });
        }
    }

    public class Item
    {
        public string Id { get; set; }
        public DateTime Date { get; set; }
        public string RefId { get; set; }
    }
}

Exception

System.Collections.Generic.KeyNotFoundException: The given key 'EmptyProjectionMember' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ProjectionMemberToIndexConvertingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.EntityShaperExpression.VisitChildren(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.VisitExtension(Expression node)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.ApplyProjection(Expression shaperExpression, ResultCardinality resultCardinality, QuerySplittingBehavior querySplittingBehavior)
   at Microsoft.EntityFrameworkCore.Query.Internal.SelectExpressionProjectionApplyingExpressionVisitor.VisitExtension(Expression extensionExpression)
   at System.Linq.Expressions.Expression.Accept(ExpressionVisitor visitor)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPostprocessor.Process(Expression query)
   at Microsoft.EntityFrameworkCore.SqlServer.Query.Internal.SqlServerQueryTranslationPostprocessor.Process(Expression query)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass12_0`1.<ExecuteAsync>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetAsyncEnumerator(CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.ConfiguredCancelableAsyncEnumerable`1.GetAsyncEnumerator()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
   at EF7_group_by_select.Program.Main() in C:\Dev\Untracked\EF7-group-by-select\Program.cs:line 13

[davidda]

Not sure if it's relevant but I wanted to mention that this worked in the old EF6. Just noticed this error coming up after migration to EF Core.

[copilotvscode]

yeah i've same issue when i need AsQueryable.

here the code:

var latestProducts = dbContext.Products
    .GroupBy(p => p.ProductCode)
    .Select(g => g.OrderByDescending(p => p.Version).FirstOrDefault())
    .Select(s => new {
    ProductCode = s.ProductCode,
    ProductName = s.ProductName,
    LatestVersion = s.Version
    });