Question

我有以下Post实体：

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;}
}

对于帖子列表，我需要检索Id，文本，评级（票数总和），CommentsCount。我尝试创建以下MapReduce索引：

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),
                          };
  }
}

我最初看起来很合理。但看起来我的地图有三个from子句将无法正常工作。我看到的唯一其他解决方案是使用带有两个映射的MultiMap索引（一个用于投票，一个用于注释）。但是使用MultiMap索引看起来有点奇怪，其中两个索引都查询同一个文档......还有其他解决方案吗？

Answer 1

Idsa，这里没有必要定义索引。这两个集合Votes和Comments都是文档的一部分，因此只需使用它们：

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
                            };

<强>更新：我确实想要预先计算结果，请看一下：http://daniellang.net/using-an-index-as-a-materialized-view-in-ravendb/

RavenDb MapReduce索引有三个来自Map的案例

1 个答案: