complete build model relathionship
complete follow all operate

1. rails generate model Relationship followed_id:integer follower_id:integer

2. User and Relationship

   *. belongs_to :followed, class_name: 'User'
   *. belongs_to :follower, class_name: 'User'

   *. has_many :relationships, foreign_key 'follower_id', dependent: 'destroy'
      has_many :followed_users, through: 'relationships', source: 'followed'

   *. has_many :reverse_relationships, foreign_key 'follower_id',
                                       class_name: 'Relationship',
                                       dependent: 'destroy'
      has_many :followers, through: 'reverse_relationships', source: 'follower'

3. Web interface -- follow/unfollow

  *. refactory home page

4. status feed

  *. Ruby + Rails + Sql