Is there a way to create a reduce index where an object key?

[MikeAlhayek]

@sebastienros It seems that the reduce index is works only with property member not an object. Is there a way to create a reduce index where an object key?

For example, this

       context.For<SearchQuerySummaryIndex, SomeKeyGroup>()
            .When(searchQuery => searchQuery.Has<SearchQueryResultPart>())
            .Map(searchQuery =>
            {
                var part = searchQuery.As<SearchQueryResultPart>();

                if (part.ContentItemIds == null || !part.ContentItemIds.Any())
                {
                    return null;
                }

                return part.ContentItemIds.Select(contentItemId => new SearchQuerySummaryIndex
                {
                    ContentItemId = contentItemId,
                    Count = 1,
                });
            }).Group(x => new SomeKeyGroup {
               ContentItemId = x.ContentItemId,
               AnotherProperty = x.AnotherProperty,
            })
            .Reduce(group => new SearchQuerySummaryIndex
            {
                ContentItemId = group.Key.ContentItemId,
                AnotherProperty  = group.Key.AnotherProperty .
                Count = group.Sum(p => p.Count)
            })
            .Delete((index, map) =>
            {
                index.Count -= map.Sum(p => p.Count);

                // if Count == 0 then delete the index
                return index.Count > 0 ? index : null;
            });

throws the following exception

Group expression is not a valid member of: