Ruby on Rails で普通にプロジェクトを作成すると sqlite3 が使われるようになっていますが、mysql を使用するにはプロジェクト作成時に -d オプションで mysql を指定します。

   1  $ rails new turntable_test -d mysql
   2        create  
   3        create  README.rdoc
   4        create  Rakefile
   5        create  config.ru
   6        create  .gitignore
   7        create  Gemfile
   8        create  app
   9        create  app/assets/javascripts/application.js
  10        create  app/assets/stylesheets/application.css
  11        create  app/controllers/application_controller.rb
  12        create  app/helpers/application_helper.rb
  13        create  app/views/layouts/application.html.erb
  14        create  app/assets/images/.keep
  15        create  app/mailers/.keep
  16        create  app/models/.keep
  17        create  app/controllers/concerns/.keep
  18        create  app/models/concerns/.keep
  19        create  bin
  20        create  bin/bundle
  21        create  bin/rails
  22        create  bin/rake
  23        create  config
  24        create  config/routes.rb
  25        create  config/application.rb
  26        create  config/environment.rb
  27        create  config/environments
  28        create  config/environments/development.rb
  29        create  config/environments/production.rb
  30        create  config/environments/test.rb
  31        create  config/initializers
  32        create  config/initializers/backtrace_silencers.rb
  33        create  config/initializers/filter_parameter_logging.rb
  34        create  config/initializers/inflections.rb
  35        create  config/initializers/mime_types.rb
  36        create  config/initializers/secret_token.rb
  37        create  config/initializers/session_store.rb
  38        create  config/initializers/wrap_parameters.rb
  39        create  config/locales
  40        create  config/locales/en.yml
  41        create  config/boot.rb
  42        create  config/database.yml
  43        create  db
  44        create  db/seeds.rb
  45        create  lib
  46        create  lib/tasks
  47        create  lib/tasks/.keep
  48        create  lib/assets
  49        create  lib/assets/.keep
  50        create  log
  51        create  log/.keep
  52        create  public
  53        create  public/404.html
  54        create  public/422.html
  55        create  public/500.html
  56        create  public/favicon.ico
  57        create  public/robots.txt
  58        create  test/fixtures
  59        create  test/fixtures/.keep
  60        create  test/controllers
  61        create  test/controllers/.keep
  62        create  test/mailers
  63        create  test/mailers/.keep
  64        create  test/models
  65        create  test/models/.keep
  66        create  test/helpers
  67        create  test/helpers/.keep
  68        create  test/integration
  69        create  test/integration/.keep
  70        create  test/test_helper.rb
  71        create  tmp/cache
  72        create  tmp/cache/assets
  73        create  vendor/assets/javascripts
  74        create  vendor/assets/javascripts/.keep
  75        create  vendor/assets/stylesheets
  76        create  vendor/assets/stylesheets/.keep
  77           run  bundle install
  78  Enter your password to install the bundled RubyGems to your system: 
  79  Fetching gem metadata from https://rubygems.org/...........
  80  Fetching gem metadata from https://rubygems.org/..
  81  Resolving dependencies...
  82  Using rake (10.1.0) 
  83  Installing i18n (0.6.9) 
  84  Using minitest (4.7.5) 
  85  Using multi_json (1.8.2) 
  86  Using atomic (1.1.14) 
  87  Using thread_safe (0.1.3) 
  88  Using tzinfo (0.3.38) 
  89  Using activesupport (4.0.1) 
  90  Using builder (3.1.4) 
  91  Using erubis (2.7.0) 
  92  Using rack (1.5.2) 
  93  Using rack-test (0.6.2) 
  94  Using actionpack (4.0.1) 
  95  Installing mime-types (1.25.1) 
  96  Using polyglot (0.3.3) 
  97  Using treetop (1.4.15) 
  98  Using mail (2.5.4) 
  99  Using actionmailer (4.0.1) 
 100  Using activemodel (4.0.1) 
 101  Using activerecord-deprecated_finders (1.0.3) 
 102  Using arel (4.0.1) 
 103  Using activerecord (4.0.1) 
 104  Using bundler (1.3.5) 
 105  Using coffee-script-source (1.6.3) 
 106  Using execjs (2.0.2) 
 107  Using coffee-script (2.2.0) 
 108  Using thor (0.18.1) 
 109  Using railties (4.0.1) 
 110  Using coffee-rails (4.0.1) 
 111  Using hike (1.2.3) 
 112  Installing jbuilder (1.5.3) 
 113  Using jquery-rails (3.0.4) 
 114  Using json (1.8.1) 
 115  Installing mysql2 (0.3.14) 
 116  Using tilt (1.4.1) 
 117  Installing sprockets (2.10.1) 
 118  Using sprockets-rails (2.0.1) 
 119  Using rails (4.0.1) 
 120  Using rdoc (3.12.2) 
 121  Using sass (3.2.12) 
 122  Using sass-rails (4.0.1) 
 123  Using sdoc (0.3.20) 
 124  Installing turbolinks (2.0.0) 
 125  Installing uglifier (2.3.2) 
 126  Your bundle is complete!
 127  Use `bundle show [gemname]` to see where a bundled gem is installed.

　オプションなしでプロジェクトを作成したときとの違いは、作成時の出力で下記のように示されている通り、mysqlのgemが使用されます。

   1  Installing mysql2 (0.3.14)

　また、sqlite3は使用されなくなるので、下記のようなsqlite3に関する出力はなくなります。

   1  Using sqlite3 (1.3.8)

　作成される config/database.yml の差分は下記の通りです。

   1  $ diff test_app/config/database.yml turntable_test/config/database.yml 
   2  1,2c1
   3  < # SQLite version 3.x
   4  < #   gem install sqlite3
   5  ---
   6  > # MySQL.  Versions 4.1 and 5.0 are recommended.
   7  4,5c3,10
   8  < #   Ensure the SQLite 3 gem is defined in your Gemfile
   9  < #   gem 'sqlite3'
  10  ---
  11  > # Install the MYSQL driver
  12  > #   gem install mysql2
  13  > #
  14  > # Ensure the MySQL gem is defined in your Gemfile
  15  > #   gem 'mysql2'
  16  > #
  17  > # And be sure to use new-style password hashing:
  18  > #   http://dev.mysql.com/doc/refman/5.0/en/old-client.html
  19  7,8c12,14
  20  <   adapter: sqlite3
  21  <   database: db/development.sqlite3
  22  ---
  23  >   adapter: mysql2
  24  >   encoding: utf8
  25  >   database: turntable_test_development
  26  10c16,18
  27  <   timeout: 5000
  28  ---
  29  >   username: root
  30  >   password:
  31  >   socket: /tmp/mysql.sock
  32  16,17c24,26
  33  <   adapter: sqlite3
  34  <   database: db/test.sqlite3
  35  ---
  36  >   adapter: mysql2
  37  >   encoding: utf8
  38  >   database: turntable_test_test
  39  19c28,30
  40  <   timeout: 5000
  41  ---
  42  >   username: root
  43  >   password:
  44  >   socket: /tmp/mysql.sock
  45  22,23c33,35
  46  <   adapter: sqlite3
  47  <   database: db/production.sqlite3
  48  ---
  49  >   adapter: mysql2
  50  >   encoding: utf8
  51  >   database: turntable_test_production
  52  25c37,39
  53  <   timeout: 5000
  54  ---
  55  >   username: root
  56  >   password:
  57  >   socket: /tmp/mysql.sock

　sqlite3 の設定が mysql 用の設定に変更されているのがわかります。username, password などは適宜変更します。
　binstubsを実行してから、rake でDBを作成します。

   1  $ bundle --binstubs
   2  $ bin/rake db:create

　mysqlにログインしてDBが作成されていることを確認します。

   1  mysql> show databases;
   2  +----------------------------+
   3  | Database                   |
   4  +----------------------------+
   5  | information_schema         |
   6  | mysql                      |
   7  | performance_schema         |
   8  | test                       |
   9  | turntable_test_development |
  10  | turntable_test_test        |
  11  +----------------------------+
  12  6 rows in set (0.00 sec)

　動作確認用のサンプルとしてUserModelを作成します。

   1  $ bin/rails generate model User name:string email:string
   2        invoke  active_record
   3        create    db/migrate/20131211224446_create_users.rb
   4        create    app/models/user.rb
   5        invoke    test_unit
   6        create      test/models/user_test.rb
   7        create      test/fixtures/users.yml

　migrate を実行してDBにテーブルを作成します

   1  $ bin/rake db:migrate
   2  ==  CreateUsers: migrating ====================================================
   3  -- create_table(:users)
   4     -> 0.0110s
   5  ==  CreateUsers: migrated (0.0111s) ===========================================

　下記のようにテーブルが作成されていれば成功です。

   1  mysql> desc users;
   2  +------------+--------------+------+-----+---------+----------------+
   3  | Field      | Type         | Null | Key | Default | Extra          |
   4  +------------+--------------+------+-----+---------+----------------+
   5  | id         | int(11)      | NO   | PRI | NULL    | auto_increment |
   6  | name       | varchar(255) | YES  |     | NULL    |                |
   7  | email      | varchar(255) | YES  |     | NULL    |                |
   8  | created_at | datetime     | YES  |     | NULL    |                |
   9  | updated_at | datetime     | YES  |     | NULL    |                |
  10  +------------+--------------+------+-----+---------+----------------+
  11  5 rows in set (0.00 sec)