Rails code to use PostGIS in place of some of Geocoder's functionality - based on http://ngauthier.com/2013/08/postgis-and-rails-a-simple-approach.html

activerecord_model_scopes.rb

# Scopes that make use of the index
def self.within_distance_of(lat:, lng:, meters:)
  where(%{
    ST_DWithin(
      ST_GeographyFromText(
        'SRID=4326;POINT(' || #{table_name}.longitude || ' ' || #{table_name}.latitude || ')'
      ),
      ST_GeographyFromText('SRID=4326;POINT(%f %f)'),
      %d
    )
  } % [lng, lat, meters])
end

def self.ordered_by_distance_from(lat:, lng:)
  order(
    %{
    ST_Distance(
      ST_GeographyFromText(
        'SRID=4326;POINT(' || #{table_name}.longitude || ' ' || #{table_name}.latitude || ')'
      ),
      ST_GeographyFromText('SRID=4326;POINT(%f %f)')
    )
    } % [lng, lat]
  )
end

migration.rb

# Migration to create index that allows us to query as if we had geography
# columns, even though we don't because Rails doesn't play nicely with those
# see http://ngauthier.com/2013/08/postgis-and-rails-a-simple-approach.html
class AddGeoQueryIndexToCmsStation < ActiveRecord::Migration
  def up
    execute %{
      create index index_cms_stations_on_geography_point ON cms_stations using gist (
        ST_GeographyFromText(
          'SRID=4326;POINT(' || cms_stations.longitude || ' ' || cms_stations.latitude || ')'
        )
      )
    }
  end

  def down
    execute %{drop index index_cms_stations_on_geography_point}
  end
end