class GeosayController < ApplicationController

  def api

    expires_in 1.minutes, :public => true

    results = Tweet.near(params[:lat].to_f,params[:lon].to_f)

    render :json => results
  end

....

class Tweet

  include MongoMapper::Document

  def self.near(lat, lon)

    self.all( :latlong  => {'$near'  => [lat, lon]}, :limit => 30) 

  end