LINQ Average conversion to SQL is correct in ver2-preview1, but incorrect in version 2

In EF Core 2.0.0-preview1-final release the Linq Average command was correctly converted to the SQL AVG command. In release 2.0.0 this seems to be broken.

If you are seeing an exception, include the full exceptions details (message and stack trace).

System.InvalidOperationException : Sequence contains no elements.
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.GetResult[TResult](IEnumerable`1 valueBuffers, Boolean throwOnNullResult)
   at lambda_method(Closure , QueryContext , ValueBuffer )
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.ProjectionShaper.TypedProjectionShaper`3.Shape(QueryContext queryContext, ValueBuffer valueBuffer)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.BufferlessMoveNext(Boolean buffer)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Ver2.UnitTests.TestAverageVer2.TakeAverageOfTwoBooks() in c:\users\jonsm\Source\Repos\EfTestDifferentVersions\Ver2\UnitTests\TestAverageVer2.cs:line 73

Steps to reproduce

I have a Book entity class with a collection of Review entity classes that contain NumVotes. I want to calculate the average number of votes. A Book can have zero to many Reviews.

My entity classes are:

public class Book                                   
{
    public int BookId { get; set; } 
    public string Title { get; set; }
    public string Description { get; set; }
    public DateTime PublishedOn { get; set; }
    public string Publisher { get; set; }
    public decimal Price { get; set; }
    public string ImageUrl { get; set; }

    //----------------------------------------------
    //relationships

    public PriceOffer Promotion { get; set; }       
    public ICollection<Review> Reviews { get; set; }
    public ICollection<BookAuthor> AuthorsLink { get; set; }                   
}

public class Review                      
{
    public int ReviewId { get; set; }
    public string VoterName { get; set; }
    public int NumStars { get; set; }
    public string Comment { get; set; }

    //-----------------------------------
    //Relationships

    public int BookId { get; set; }      
}

My test code is below, and it fails on the database query when running EF Core 2.0.0, but works correctly if running EF Core 2.0.0-preview1-final. Note: the method TwoBooksOneWithReviews creates two Book entities; one with no reviews and the second with two reviews.

private class Dto
{
    public string Title { get; set; }
    public double? AveVotes { get; set; }
}

[Fact]
public void TakeAverageOfTwoBooks()
{
    //SETUP
    var options = SqliteInMemory.CreateOptions<EfCoreContext>();

    using (var context = new EfCoreContext(options))
    {
        context.Database.EnsureCreated();

        context.AddRange(SeedData.TwoBooksOneWithReviews());
        context.SaveChanges();
        var logs = context.SetupLogging();

        //VERIFY
        var result = context.Books.Select(x => new Dto
        {
            Title = x.Title,
            AveVotes = x.Reviews.Select(y => y.NumStars).Average()
        }).ToList();

        //VERIFY
        result.First().AveVotes.ShouldBeNull();
        result.Last().AveVotes.ShouldEqual(4);
        foreach (var log in logs)
        {
            _output.WriteLine(log);
        }
    }
}

If I run exactly the same test on release 2.0.0-preview1-final then it works correctly and outputs the following SQL

Executed DbCommand (0ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT "x"."Title", (
    SELECT AVG(CAST("y"."NumStars" AS REAL))
    FROM "Review" AS "y"
    WHERE "x"."BookId" = "y"."BookId"
) AS "AveVotes"
FROM "Books" AS "x"

Further technical details

EF Core version: 2.0.0 incorrect (2.0.0-preview1-final correct) Database Provider: Microsoft.EntityFrameworkCore.Sqlite (but fails on SqlServer too) Operating system: Windows 10 IDE: VS2017 15.3

[JonPSmith]

Could I ask if this bug is fixed in the planned patch update, 2.0.1? My reasons for asking this are:

1. It seems that you know what the problem is - DetermineAggregateThrowingBehavior. I therefore assume its not hard to fix.
2. The SQL code produced is very poor, and you get a lot of warnings in the log.
3. I am trying to release the book Entity Framework in Action soon and I include Average as an example in the book - see demo web site http://efcoreinaction.com/ (currently running EF Core 1.1.0).

[ajcvickers]

@anpete to follow up with a fix and we can discuss whether to patch or not.