[Exec] rails generate model Micropost content:string user_id:integer
[Add] The Micropost migration. （Note the index on user_id and created_at.）
[Add] The initial Micropost spec.
[Exec] bundle exec rake db:migrate
[Add] Tests for the validity of a new micropost.
[Add] A validation for the micropost's user_id.
[Add] A test to ensure that the user_id isn't accessible.
[Mod] Making the content attribute（and only the content attribute）accessible.
[Add] Tests for the micropost's user association.
[Add] A test for the user's microposts attribute.
[Add] A micropost belongs_to a user and a user has_many microposts.
[Add] The complete factory file, including a new factory for microposts.
[Add] Testing the order of a user's microposts.
[Add] Ordering the microposts with default_scope.
[Add] Testing that microposts are destroyed when users are.
[Add] Ensuring that a user's microposts are destroyed along with the user.
[Add] Tests for the Micropost model validations.
[Add] The Micropost model validations.
[Add] A test for showing microposts on the user show page.
[Add] Adding microposts to the user show page.