Question

Given a model like this:

public class Location
{
    public int Id { get; set; }

    public string Name { get; set; }

    public Coordinates Geo { get; set; }

    public IList<Review> Reviews { get; set; }
}

public class Review
{
    public string Comment { get; set; }

    public int Rating { get; set; }
}

How can I write a query that will return matching locations sorted in descending order by their Rating?

I have an index that allows me to query for locations within a certain radius around a geopoint:

public class Locations_ByCoordinates : AbstractIndexCreationTask<Location>
{
    public Locations_ByCoordinates()
    {
        Map = locations => from l in locations
                           select new {_ = SpatialIndex.Generate(l.Geo.Lat, l.Geo.Lon)};
    }
}

This seems to work and I am able to sort the results like so:

RavenQueryStatistics stats;
var query = session.Advanced.LuceneQuery<Location>(Names.Indexes.LocationsByCoordinates)
                       .Statistics(out stats)
                       .WithinRadiusOf(radius, coordinates.Lat, coordinates.Lon);

switch (sort)
{
    case Sort.Distance:
        query = query.SortByDistance();
        break;
    case Sort.Rating:
        query = query.AddOrder(l => l.Reviews.Average(r => r.Rating), true); // TODO: This throws an exception, perhaps a modified index with an aggregate is required? Maybe Map/Reduce?
        break;
    case Sort.Alphabetical:
        query = query.OrderBy(l => l.Name);
        break;
    case Sort.Number_of_ratings:
        query = query.AddOrder(l => l.Reviews.Count, true);
        break;
    case Sort.Relevance: // This is the default ordering in RavenDB.
        break;
    default:
        throw new ArgumentOutOfRangeException("sort");
}

Every option works OK except sorting by average rating which throws an exception about unknown field. I feel this has to be a Map/Reduce index rather than a Map index. But I have no idea how to write it.

I am migrating this code from MongoDB, but I used to sort the documents by rating on the client side there, I thought I’d do this on the server side now.

Answer 1

Since all reviews are in the location, and you want the average rating of the reviews for each location, you don’t need to reduce. Just create a new field in your map.

Map = locations => from l in locations
                   select new {
                       _ = SpatialIndex.Generate(l.Geo.Lat, l.Geo.Lon),
                       AvgRating = l.Reviews.Average(r => r.Rating)
                   };