How to perform spatial joins in ravendb - join

Question

I have a large collection of documents containing geospatial point data (among other data). I have another large collection of documents containing polygons (among other data).
I want to filter queries on the point data by whether the points are included in any of the polygons that have a certain property.
Can I do this with RavenDB and if so, how?
Things I thought about:
I can't see how I could do this with an index because indexes only map (and/or reduce), so I can not query one collection by another.
I can't just make the query and rely on Raven's result caching, because querying by the set of polygons would quickly make the query length exceed any sensible query length limit.

Answer 1

In RavenDB you can define indexes to deal with spatial data.
Let's assume you have a document with spatial polygon defined as WKT:
(WKT --> Well Known Text markup https://en.wikipedia.org/wiki/Well-known_text)
public class EventWithWKT
{
public string Id { get; set; }
public string Name { get; set; }
public string WKT { get; set; }
}
By polygon WKT I mean something like
POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))
Then you can define index that would handle the WKT like this:
public class EventsWithWKT_ByNameAndWKT : AbstractIndexCreationTask<EventWithWKT>
{
public EventsWithWKT_ByNameAndWKT()
{
Map = events => from e in events
select new
{
Name = e.Name,
WKT = e.WKT
};
Spatial(x => x.WKT, options => options.Geography.Default());
}
}
By calling the "Spatial" method in index definition, RavenDB creates special column that can be then queried with spatial queries.
Now filtering on whether a certain point is within the polygons of the documents or not, can be done with the following query:
var results = session
.Query<EventWithWKT, EventsWithWKT_ByNameAndWKT>()
.Customize(x => x.RelatesToShape("WKT", "POINT (30 10)", SpatialRelation.Within))
.ToList();
That is not the only way to define and use spatial data. You can read more about RavenDB spatial indexes in those documentation articles:
defining spatial indexes
querying spatial indexes