Convert from MongoDB geolocation finder to raw SQL query

app/models/post.rb

@@ -1,13 +1,51 @@
 class Post < ActiveRecord::Base
   belongs_to :feed
   
-  EARTH_RADIUS_M = 6378000
+  EARTH_RADIUS_METRES = 6378000
   
-  def self.near(lat, lng, radius_m)
-    all(
-      :loc => {
-        '$nearSphere' => [ lng, lat ],
-        '$maxDistance' => radius_m / EARTH_RADIUS_M
-    })
+  def self.near(lat, lon, radius_metres)
+    # Santize inputs. Is this necessary?
+    lat = lat.to_f
+    lon = lon.to_f
+    radius_metres = radius_metres.to_i
+
+    # Calculate distance using the Haversine formula  
+    self.find_by_sql(<<-ENDQUERY
+      SELECT
+        p.id,
+        p.title,
+        p.summary,
+        p.url,
+        p.lat,
+        p.lon,
+        p.published,
+        f.title as feed_title,
+        ( #{EARTH_RADIUS_METRES}
+        * acos( cos( radians('#{lat}') )
+        * cos( radians( p.lat ) )
+        * cos( radians( p.lon )
+        - radians('#{lon}') )
+        + sin( radians('#{lat}') )
+        * sin( radians( p.lat ) ) ) )
+        As distance
+
+      FROM posts p
+      INNER JOIN feeds f
+      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 ) ) ) )
+        < #{radius_metres}
+        
+      ORDER BY distance
+
+      ENDQUERY
+    )
   end
 end