nearest_places_with_mongo.rb

      lat, lng = [53, 21]
      conditions = { :page => 1, :per_page => 20 }
      map = "function() { emit(this._id, {lat: this.lat, lng: this.lng}); }"
      reduce = <<BEGIN
function(key, values) {
  distance = 0;
  values.forEach(function(doc) {
   distance = (doc.lat - #{lat})*(doc.lat - #{lat}) + (doc.lng - #{lng})*(doc.lng - #{lng});
  });
  return distance;
};
BEGIN
    results = Place.collection.map_reduce(map, reduce).find({}, {:sort => "value ASC", :limit => conditions[:per_page]})
    conditions[:_id] = { "$in" => results.collect { |r| r["_id"]} }
    end

    Place.paginate(conditions)