Allow Default for SortFieldDescriptor

[markwalsh-liverpool]

When creating a SortFieldDescriptor it should be possible to specific default (relevance) and have that ignored when creating a SearchDescriptor, in my specific example, I build up a sort descriptor independently based on a string

 private static SortFieldDescriptor<ElasticVideo> GetDocumentSortQuery(ElasticSearchVideoParameters searchParameters)
        {
            var sortFieldDescriptor = new SortFieldDescriptor<ElasticVideo>();

            switch (searchParameters.SortBy)
            {
                case SearchSortOrder.Relevance:
                    return null;
                case SearchSortOrder.CreatedDateDesc:
                    sortFieldDescriptor.Descending().OnField("CreatedDate");
                    break;
                case SearchSortOrder.CreatedDateAsc:
                    sortFieldDescriptor.Ascending().OnField("CreatedDate");
                    break;
                case SearchSortOrder.TitleDesc:
                    sortFieldDescriptor.Descending().OnField("Title.raw");
                    break;
                case SearchSortOrder.TitleAsc:
                    sortFieldDescriptor.Ascending().OnField("Title.raw");
                    break;
                case SearchSortOrder.LikesDesc:
                    sortFieldDescriptor.Descending().OnField("LikesCount");
                    break;
                case SearchSortOrder.LikesAsc:
                    sortFieldDescriptor.Ascending().OnField("LikesCount");
                    break;
                case SearchSortOrder.DislikesDesc:
                    sortFieldDescriptor.Descending().OnField("DislikeCount");
                    break;
                case SearchSortOrder.DislikesAsc:
                    sortFieldDescriptor.Ascending().OnField("DislikeCount");
                    break;
                case SearchSortOrder.ViewsDesc:
                    sortFieldDescriptor.Descending().OnField("ViewCount");
                    break;
                case SearchSortOrder.ViewsAsc:
                    sortFieldDescriptor.Ascending().OnField("ViewCount");
                    break;
            }

            return sortFieldDescriptor;
        }

So when it comes to use this built descriptor I have to use an if statement

            if (documentSortQuery != null)
                searchDescriptor
                    .AllIndices()
                    .Type("video")
                    .Query(documentQuery)
                    .Sort(x => documentSortQuery)
                    .Skip(searchParameters.Skip)
                    .Take(searchParameters.Take);
            else
            {
                searchDescriptor
                    .AllIndices()
                    .Type("video")
                    .Query(documentQuery)
                    .Skip(searchParameters.Skip)
                    .Take(searchParameters.Take);
           }

Not sure if there is a more elegant way. Please let me know if I just have to "deal with it". If you think it's valuable adding this, I wouldn't mind having a crack at a PR.

[Mpdreamz]

You are absolutely correct, returning null should always be OK inside descriptors. We are currently reworking our test infrastructure from scratch and this will be one of the things we'll test for more automatically starting with NEST 2.0.

Thanks for reporting!

[Mpdreamz]

Until a new version goes out where this is fixed you could "fix" it by returning

return sortFieldDescriptor.OnField("_score");

To get rid of that ugly if statement 😄

[markwalsh-liverpool]

Thanks very much will do. Damn it, would have loved to of done the PR!