location.rb

class Location < ActiveRecord::Base
  DEFAULT_OPTS = {:units => :km}

  class << self
    def near(coords, radius = 10, opts = {})
      options = DEFAULT_OPTS.merge(options)
      radius = radius.to_i
      # do stuff with coords and radius and options
    end
  end
end

locations_controller.rb

  def index
    @locations = if params[:near]
                   Location.near(params[:near], params[:radius] || 10)
                 else
                   Location.all
                 end
  end