Only query for posts on the specified layer

app/models/post.rb

@@ -3,11 +3,12 @@ class Post < ActiveRecord::Base
   
   EARTH_RADIUS_METRES = 6378000
   
-  def self.near(lat, lon, radius_metres)
+  def self.near(lat, lon, radius_metres, layer_id)
     # Santize inputs. Is this necessary?
     lat = lat.to_f
     lon = lon.to_f
     radius_metres = radius_metres.to_i
+    layer_id = layer_id.to_i
 
     # Calculate distance using the Haversine formula  
     self.find_by_sql(<<-ENDQUERY
@@ -34,13 +35,31 @@ class Post < ActiveRecord::Base
       ON p.feed_id = f.id
       
       WHERE
-        ( #{EARTH_RADIUS_METRES}
-        * acos( cos( radians('#{lat}') )
-        * cos( radians( p.lat ) )
-        * cos( radians( p.lon )
-        - radians('#{lon}') )
-        + sin( radians('#{lat}') )
-        * sin( radians( p.lat ) ) ) )
+        p.id IN
+        ( -- Subquery returns a list of post_ids for posts on this layer
+          SELECT p.id
+
+          FROM feeds_layers fl
+          
+          INNER JOIN feeds f
+          ON fl.feed_id = f.id
+
+          INNER JOIN posts p
+          ON p.feed_id = f.id
+
+          WHERE fl.layer_id = #{layer_id}
+        )
+          
+        AND
+        (
+          #{EARTH_RADIUS_METRES}
+          * acos( cos( radians('#{lat}') )
+          * cos( radians( p.lat ) )
+          * cos( radians( p.lon )
+          - radians('#{lon}') )
+          + sin( radians('#{lat}') )
+          * sin( radians( p.lat ) ) )
+        )
         <= #{radius_metres}
         
       ORDER BY distance