Note: This post is intended as a supplement to WTF Just Happened? A Quick Tour of Your First Rails App.

Database Migrations

When we need to make changes to our database schema (adding or modifying a table), we do so with an Active Record migration. A migration file is provided for you when you run rails generate scaffold Post, or you can create your own by running rails generate migration.

Below is the migration file that was created for us when we ran rails generate scaffold Post title:string body:text.

class CreatePosts < ActiveRecord::Migration[5.0]
  def change
    create_table :posts do |t|
      t.string :title
      t.text :body

      t.timestamps
    end
  end
end

The migration file is named 20170124022430_create_posts.rb. Rails named it create_posts because that is descriptive of it's behaviour. It appended a datetime stamp so we can retrieve all migration files in the order in which they were created. Should Rails ever need to revert the database to a point in time, it can do so because we have this timeline built into the naming of the migration files.

When a migration is run (with the command rails db:migrate), the database schema is updated. Details of the schema can be found in the file app/db/schema.rb.

Below is the contents of schema.rb after we have run rails generate scaffold Post title:string body:text:

# This file is auto-generated from the current state of the database. Instead
# of editing this file, please use the migrations feature of Active Record to
# incrementally modify your database, and then regenerate this schema definition.
#
# Note that this schema.rb definition is the authoritative source for your
# database schema. If you need to create the application database on another
# system, you should be using db:schema:load, not running all the migrations
# from scratch. The latter is a flawed and unsustainable approach (the more migrations
# you'll amass, the slower it'll run and the greater likelihood for issues).
#
# It's strongly recommended that you check this file into your version control system.

ActiveRecord::Schema.define(version: 20170124022430) do

  create_table "posts", force: :cascade do |t|
    t.string   "title"
    t.text     "body"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

end

You will note that, in addition to the specified body and title attributes, Rails has provided us with two datetime stamps; created_at and updated_at. These record when the record was first created, and when it was last updated. Rails manages these values for us.