Support MemberInit expression in group by

[Molinware]

Execute this query gives the following exception:

public class Company
{
    public int Id { get; set; }
    public string Description { get; set; }
}

public class CompanyDto
{
    public int Id { get; set; }
    public string Description { get; set; }
}

var query = session.Query<Company>()
    .Select(x =>
        new CompanyDto()
	{
	    Id = x.Id,
	    Description = x.Description,
	})
    .GroupBy(x => x.Id)
    .Select(x =>
        new CompanyDto()
	{
	    Id = x.Key,
	    Description = x.Max(y => y.Description),
	});

query.ToList()

The method or operation is not implemented
[StackTrace]
in NHibernate.Linq.GroupBy.GroupBySelectClauseRewriter.VisitMember(MemberExpression expression)
in NHibernate.Linq.Expressions.NhAggregatedExpression.VisitChildren(ExpressionVisitor visitor)
in NHibernate.Linq.Expressions.NhExpression.Accept(ExpressionVisitor visitor)
in NHibernate.Linq.GroupBy.GroupBySelectClauseRewriter.ReWrite(Expression expression, GroupResultOperator groupBy, QueryModel model)
in NHibernate.Linq.GroupBy.GroupBySelectClauseRewriter.VisitSubQuery(SubQueryExpression expression)
in System.Linq.Expressions.ExpressionVisitor.VisitMemberAssignment(MemberAssignment node)
in System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor)
in System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node)
in NHibernate.Linq.GroupBy.GroupBySelectClauseRewriter.ReWrite(Expression expression, GroupResultOperator groupBy, QueryModel model)
in NHibernate.Linq.GroupBy.AggregatingGroupByRewriter.<>c__DisplayClass2_0.<FlattenSubQuery>b__0(Expression s)
in Remotion.Linq.Clauses.SelectClause.TransformExpressions(Func`2 transformation)
in NHibernate.Linq.GroupBy.AggregatingGroupByRewriter.FlattenSubQuery(QueryModel queryModel, QueryModel subQueryModel, GroupResultOperator groupBy)
in NHibernate.Linq.GroupBy.AggregatingGroupByRewriter.ReWrite(QueryModel queryModel)
in NHibernate.Linq.Visitors.QueryModelVisitor.GenerateHqlQuery(QueryModel queryModel, VisitorParameters parameters, Boolean root, Nullable`1 rootReturnType)
in NHibernate.Linq.NhLinqExpression.Translate(ISessionFactoryImplementor sessionFactory, Boolean filter)
in NHibernate.Hql.Ast.ANTLR.ASTQueryTranslatorFactory.CreateQueryTranslators(IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 filters, ISessionFactoryImplementor factory)
in NHibernate.Engine.Query.QueryExpressionPlan.CreateTranslators(IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
in NHibernate.Engine.Query.QueryExpressionPlan..ctor(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory)
in NHibernate.Engine.Query.QueryPlanCache.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters)
in NHibernate.Impl.AbstractSessionImpl.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow)
in NHibernate.Impl.AbstractSessionImpl.CreateQuery(IQueryExpression queryExpression)
in NHibernate.Linq.DefaultQueryProvider.PrepareQuery(Expression expression, IQuery& query)
in NHibernate.Linq.DefaultQueryProvider.Execute(Expression expression)
in NHibernate.Linq.DefaultQueryProvider.Execute[TResult](Expression expression)
in Remotion.Linq.QueryableBase`1.GetEnumerator()
in System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
in System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)

[Molinware]

It looks like making a small change on the "GroupBySelectClauseRewriter" class resolves the problem, but i don't know what problems can it cause:

protected override Expression VisitMember(MemberExpression expression)
{
    if (!IsMemberOfModel(expression))
    {
        return base.VisitMember(expression);
    }

    if (expression.IsGroupingKeyOf(_groupBy))
    {
        // If we have referenced the Key, then return the nominated key expression
	return _nominatedKeySelector;
    }

    var elementSelector = _groupBy.ElementSelector;

    if ((elementSelector is MemberExpression) || (elementSelector is QuerySourceReferenceExpression))
    {
        // If ElementSelector is MemberExpression, just return
        return base.VisitMember(expression);
    }

    if ((elementSelector is NewExpression || elementSelector.NodeType == ExpressionType.Convert)
        && elementSelector.Type == expression.Expression.Type)
    {
        //TODO: probably we should check this with a visitor
        return Expression.MakeMemberAccess(elementSelector, expression.Member);
    }

    throw new NotImplementedException();
}

Change:

throw new NotImplementedException();

To:

return Expression.MakeMemberAccess(elementSelector, expression.Member);

[Molinware]

So... It's been 5 months...

[fredericDelaporte]

This is open-source and volunteer based work. You are welcome to contribute a PR, following contributing guidelines.
Otherwise you will have to wait for someone having time to handle this.

[fairking]

Change: throw new NotImplementedException();
To: return Expression.MakeMemberAccess(elementSelector, expression.Member);

I tried to add this line on the latest master and ended up with another error by executing odata request '/odata/tickets?$apply=groupby((priority))':

System.NotSupportedException
  HResult=0x80131515
  Message=MemberInit
  Source=NHibernate
  StackTrace:
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.VisitExpression(Expression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 149
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.<VisitNewArrayExpression>b__35_0(Expression exp) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 590
   at NHibernate.Util.EnumerableExtensions.ToArray[TInput,TOutput](ICollection`1 input, Func`2 converter) in C:\Code\nhibernate-core\src\NHibernate\Util\EnumerableExtensions.cs:line 49
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.VisitNewArrayExpression(NewArrayExpression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 590
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.VisitExpression(Expression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 103
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.Visit(Expression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 42
   at NHibernate.Linq.Visitors.HqlGeneratorExpressionVisitor.Visit(Expression expression, VisitorParameters parameters) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\HqlGeneratorExpressionVisitor.cs:line 28
   at NHibernate.Linq.Visitors.ResultOperatorProcessors.ProcessGroupBy.<>c__DisplayClass0_0.<Process>b__0(Expression k) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\ResultOperatorProcessors\ProcessGroupBy.cs:line 19
   at System.Linq.Enumerable.SelectArrayIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at NHibernate.Linq.Visitors.ResultOperatorProcessors.ProcessGroupBy.Process(GroupResultOperator resultOperator, QueryModelVisitor queryModelVisitor, IntermediateHqlTree tree) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\ResultOperatorProcessors\ProcessGroupBy.cs:line 21
   at NHibernate.Linq.Visitors.ResultOperatorProcessors.ResultOperatorProcessor`1.Process(ResultOperatorBase resultOperator, QueryModelVisitor queryModel, IntermediateHqlTree tree) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\ResultOperatorProcessors\ResultOperatorProcessor.cs:line 17
   at NHibernate.Linq.Visitors.ResultOperatorProcessors.ResultOperatorMap.Process(ResultOperatorBase resultOperator, QueryModelVisitor queryModel, IntermediateHqlTree tree) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\ResultOperatorProcessors\ResultOperatorMap.cs:line 24
   at NHibernate.Linq.Visitors.QueryModelVisitor.VisitResultOperator(ResultOperatorBase resultOperator, QueryModel queryModel, Int32 index) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\QueryModelVisitor.cs:line 379
   at Remotion.Linq.Clauses.ResultOperatorBase.Accept(IQueryModelVisitor visitor, QueryModel queryModel, Int32 index)
   at Remotion.Linq.QueryModelVisitorBase.VisitResultOperators(ObservableCollection`1 resultOperators, QueryModel queryModel)
   at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
   at NHibernate.Linq.Visitors.QueryModelVisitor.Visit() in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\QueryModelVisitor.cs:line 163
   at NHibernate.Linq.Visitors.QueryModelVisitor.GenerateHqlQuery(QueryModel queryModel, VisitorParameters parameters, Boolean root, Nullable`1 rootReturnType) in C:\Code\nhibernate-core\src\NHibernate\Linq\Visitors\QueryModelVisitor.cs:line 104
   at NHibernate.Linq.NhLinqExpression.Translate(ISessionFactoryImplementor sessionFactory, Boolean filter) in C:\Code\nhibernate-core\src\NHibernate\Linq\NhLinqExpression.cs:line 84
   at NHibernate.Hql.Ast.ANTLR.ASTQueryTranslatorFactory.CreateQueryTranslators(IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 filters, ISessionFactoryImplementor factory) in C:\Code\nhibernate-core\src\NHibernate\Hql\Ast\ANTLR\ASTQueryTranslatorFactory.cs:line 19
   at NHibernate.Engine.Query.QueryExpressionPlan.CreateTranslators(IQueryExpression queryExpression, String collectionRole, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory) in C:\Code\nhibernate-core\src\NHibernate\Engine\Query\QueryExpressionPlan.cs:line 38
   at NHibernate.Engine.Query.QueryExpressionPlan..ctor(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters, ISessionFactoryImplementor factory) in C:\Code\nhibernate-core\src\NHibernate\Engine\Query\QueryExpressionPlan.cs:line 20
   at NHibernate.Engine.Query.QueryPlanCache.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow, IDictionary`2 enabledFilters) in C:\Code\nhibernate-core\src\NHibernate\Engine\Query\QueryPlanCache.cs:line 63
   at NHibernate.Impl.AbstractSessionImpl.GetHQLQueryPlan(IQueryExpression queryExpression, Boolean shallow) in C:\Code\nhibernate-core\src\NHibernate\Impl\AbstractSessionImpl.cs:line 539
   at NHibernate.Impl.AbstractSessionImpl.CreateQuery(IQueryExpression queryExpression) in C:\Code\nhibernate-core\src\NHibernate\Impl\AbstractSessionImpl.cs:line 507
   at NHibernate.Linq.DefaultQueryProvider.PrepareQuery(Expression expression, IQuery& query) in C:\Code\nhibernate-core\src\NHibernate\Linq\DefaultQueryProvider.cs:line 191
   at NHibernate.Linq.DefaultQueryProvider.Execute(Expression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\DefaultQueryProvider.cs:line 93
   at NHibernate.Linq.DefaultQueryProvider.Execute[TResult](Expression expression) in C:\Code\nhibernate-core\src\NHibernate\Linq\DefaultQueryProvider.cs:line 100
   at Remotion.Linq.QueryableBase`1.System.Collections.IEnumerable.GetEnumerator()
   at System.Text.Json.JsonSerializer.HandleEnumerable(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state)
   at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 originalWriterDepth, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.<WriteAsyncCore>d__50.MoveNext()