Rails4でGoogleMapを表示させる ref: http://qiita.com/jacoyutorius/items/a107ff6c93529b6b393e

application.html.erb

<script src="//maps.google.com/maps/api/js?v=3.13&amp;sensor=false&amp;libraries=geometry" type="text/javascript"></script>
<script src='//google-maps-utility-library-v3.googlecode.com/svn/tags/markerclustererplus/2.0.14/src/markerclusterer_packed.js' type='text/javascript'></script>

application.js

//= require underscore
//= require gmaps/google

file0.txt

rails new gmap 
cd gmap

file2.txt

bundle install

file3.txt

rails g scaffold user title:string description:string address:string latitude:float longitude:float
rake db:migrate

file8.txt

rails g controller map index

Gemfile

gem "gmaps4rails"
gem "geocoder"

index.html.erb

<div style='width: 800px;'>
  <div id="map" style='width: 800px; height: 400px;'></div>
</div>

<script type="text/javascript">
    handler = Gmaps.build('Google');
    handler.buildMap({ provider: {}, internal: {id: 'map'}}, function(){
      markers = handler.addMarkers(<%=raw @hash.to_json %>);
      handler.bounds.extendWith(markers);
      handler.fitMapToBounds();
    });
</script>

map_controller.rb

class MapController < ApplicationController
  def index
    @users = User.all
    @hash = Gmaps4rails.build_markers(@users) do |user, marker|
      marker.lat user.latitude
      marker.lng user.longitude
      marker.infowindow user.description
      marker.json({title: user.title})
    end
  end
end

underscore.js

(省略)

user.rb

class User < ActiveRecord::Base
  geocoded_by :address
  after_validation :geocode
end