Added geospatial index and search

app/models/post.rb

@@ -10,5 +10,17 @@ class Post
   key :loc,         Hash # { lng, lat }
   timestamps!
 
+  ensure_index [[:loc, '2d']]
+
   belongs_to :feed
+  
+  EARTH_RADIUS_KM = 6378.0
+  
+  def self.near(lat, lng, radius_km)
+    all(
+      :loc => {
+        '$nearSphere' => [ lng, lat ],
+        '$maxDistance' => radius_km / EARTH_RADIUS_KM
+    })
+  end
 end