Question

I have the following Post entity:

public class Post
{
   public string Id {get;set;}
   public string Text {get;set;}
   public IList<Vote> Votes {get;set;}
   public IList<Comment> Comments {get;set;}
}

For list of Posts I need to retrieve Id, Text, Rating (sum of votes), CommentsCount. I tried to create the following MapReduce index:

public class PostsForList: AbstractIndexCreationTask<Post, PostsForList.ReduceResult>
{
  public class ReduceResult
  {
    public string Id { get; set; }
    public string Text { get; set; }
    public long Rating { get; set; }
    public long CommentsCount { get; set; }
  }

  public PostsForList()
  {
    Map = posts => from post in posts
                              from comment in post.Comments
                              from vote in post.Votes
                              select
                                new
                                {
                                  Id = post.Id,
                                  Text = post.Text,
                                  Rating = vote.Value /* 1 or -1 */
                                  CommentsCount = 1,
                                };

    Reduce = results => from result in results
                        group result by result.Id
                        into grouped
                        select
                          new
                          {
                            Id = grouped.Key,
                            Text = grouped.Select(x => x.Text).First(),
                            Rating = grouped.Sum(x => x.Rating)
                            CommentsCount = grouped.Sum(x => x.Rating),
                          };
  }
}

It looked reasonable for me initially. But looks like my Map with three from clauses won’t work. The only other solution I see is to use MultiMap index with two maps (one for votes and one for comments). But it looks a bit strange to use MultiMap index where both indexes query the same document… Are there any other solutions?

Answer 1

Idsa, there’s no need to define an index here. Both collections, Votes and Comments are part of the document, so just use them:

var posts = documentSession.Query<Post>()
    .Skip(CurrentPage*PageSize)
    .Take(PageSize)
    .ToList(); // here is the db-call! all following is just in-memory

var viewModelPosts = from post in posts
                        select new
                            {
                                post.Id,
                                post.Text,
                                Rating = post.Votes.Sum(x => x.Value),
                                CommentsCount = post.Comments.Count
                            };

Update:
I you really want to precompute the results, take a look here: http://daniellang.net/using-an-index-as-a-materialized-view-in-ravendb/