Indexing list count within last month - ravendb

Question

I would like to be able to query the first 10 documents from a collection in RavenDB ordered by the count with a constraint in a sublist. This is my entity:
public class Post
{
public string Title { get; set; }
public List<Like> Likes { get; set; }
}
public class Like
{
public DateTime Created { get; set; }
}
I've tried with the following query:
var oneMonthAgo = DateTime.Today.AddMonths(-1);
session
.Query<Post>()
.OrderByDescending(x => x.Likes.Count(y => y.Created > oneMonthAgo))
.Take(10);
Raven complaints that count should be done on index time rather than query time. I've tried moving the count to a index using the following code:
public class PostsIndex : AbstractIndexCreationTask<Post>
{
public PostsIndex()
{
var month = DateTime.Today.AddMonths(-1);
Map = posts => from doc in posts
select
new
{
doc.Title,
LikeCount = doc.Likes.Count(x => x.Created > month),
};
}
}
When adding this index, Raven throws a error 500.
What to do?

Answer 1

You can do this by creating a Map/Reduce index to flatten the Posts/Likes and then query over that.
The index:
public class PostLikesPerDay : AbstractIndexCreationTask<Post, PostLikesPerDay.Result>
{
public PostLikesPerDay()
{
Map = posts => from post in posts
from like in post.Likes
select new Result
{
Title = post.Title,
Date = like.Created,
Likes = 1
};
Reduce = results => from result in results
group result by new
{
result.Title,
result.Date.Date
}
into grp
select new Result
{
Title = grp.Key.Title,
Date = grp.Key.Date,
Likes = grp.Sum(l => l.Likes)
};
}
public class Result
{
public string Title { get; set; }
public DateTime Date { get; set; }
public int Likes { get; set; }
}
}
And the query:
using (var session = store.OpenSession())
{
var oneMonthAgo = DateTime.Today.AddMonths(-1);
var query = session.Query<PostLikesPerDay.Result, PostLikesPerDay>()
.Where(y => y.Date > oneMonthAgo)
.OrderByDescending(p => p.Likes)
.Take(10);
foreach (var post in query)
{
Console.WriteLine("'{0}' has {1} likes on {2:d}", post.Title, post.Likes, post.Date);
}
}