Query: GroupJoin and SelectMany fails to translate

[Dixin]

I am porting EF code to EF Core, and got this error. GroupJoin returns hierarchical data, so use SelectMany to flatten it:

var leftOuterJoin = adventureWorks.Products
    .GroupJoin(
        inner: adventureWorks.Transactions,
        outerKeySelector: product => product.ProductID,
        innerKeySelector: transaction => transaction.ProductID,
        resultSelector: (product, transactions) => new
        {
            Product = product,
            Transactions = transactions
        })
    .SelectMany(
        productTransactions => productTransactions.Transactions,
        (productTransactions, transaction) => new
        {
            Name = productTransactions.Product.Name,
            ActualCost = transaction.ActualCost
        });

In EF, the translation is correct:

SELECT 
    [Extent1].[ProductCategoryID] AS [ProductCategoryID], 
    [Extent1].[Name] AS [Name], 
    [Extent2].[Name] AS [Name1]
    FROM  [Production].[ProductCategory] AS [Extent1]
    LEFT OUTER JOIN [Production].[ProductSubcategory] AS [Extent2] ON [Extent1].[ProductCategoryID] = [Extent2].[ProductCategoryID]

In EF Core, the translation fails, and throws exception:

{System.InvalidOperationException: No coercion operator is defined between types 'ConsoleApp1.ProductSubcategory' and 'System.String'.
at System.Linq.Expressions.Expression.GetUserDefinedCoercionOrThrow(ExpressionType coercionType, Expression expression, Type convertToType)
at Remotion.Linq.Parsing.RelinqExpressionVisitor.d__0.MoveNext()
at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source, Int32& length)
at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
at System.Dynamic.Utils.CollectionExtensions.ToReadOnly[T](IEnumerable`1 enumerable)
at System.Linq.Expressions.Expression.New(ConstructorInfo constructor, IEnumerable`1 arguments, IEnumerable`1 members)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.<>c__DisplayClass67_0.b__0(Expression e) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 749
at Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
at Remotion.Linq.QueryModel.TransformExpressions(Func`2 transformation)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.OptimizeJoinClause(JoinClause joinClause, QueryModel queryModel, Int32 index, Action baseVisitAction, MethodInfo operatorToFlatten, Boolean groupJoin) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 748
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitGroupJoinClause(GroupJoinClause groupJoinClause, QueryModel queryModel, Int32 index) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 623
at Remotion.Linq.QueryModelVisitorBase.VisitBodyClauses(ObservableCollection`1 bodyClauses, QueryModel queryModel)
at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.VisitQueryModel(QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Query\EntityQueryModelVisitor.cs:line 721
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitQueryModel(QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 373
at Microsoft.EntityFrameworkCore.Query.Internal.SqlServerQueryModelVisitor.VisitQueryModel(QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.SqlServer\Query\Internal\SqlServerQueryModelVisitor.cs:line 92
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Query\EntityQueryModelVisitor.cs:line 259
at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Storage\Database.cs:line 67}

The exception is from RelationalQueryModelVisitor.OptimizeJoinClause:

queryModel.TransformExpressions(e =>
    ReferenceReplacingExpressionVisitor
        .ReplaceClauseReferences(
            e,
            querySourceMapping,
            throwOnUnmappedReferences: false));

[maumar]

This works in current bits. We produce the following sql:

SELECT [product].[Name], [transaction].[ActualCost]
FROM [Products] AS [product]
INNER JOIN [Transactions] AS [transaction] ON [product].[ProductID] = [transaction].[ProductID]