Migrations

Overview

A way to evolve your database schema over time
Migrations use a database independent Ruby API
script/generate migration (creates migration file)
Each migration file is prefixed by a unique time stamp
Migration classes extend ActiveRecord::Migration and have an up and a down method
Rails keeps track of which migrations have been run in the schema_migrations table
rake db:migrate (runs new migrations)

Managing Tables and Columns

create_table, add_column, change_column, rename_column, rename_table, add_index
Column types: binary, boolean, date, datetime, decimal, float, integer, string, text, time, timestamp
Column options: :null, :limit, :default
Table options: :primary_key, :id, :force, :options
Execute custom SQL with execute(“any SQL you like here”)

Things to be Aware of

You can use ActiveRecord classes in migrations, but this is fragile as the class definitions might change over time
Foreign keys you have to create yourself. You can use a helper module or a plugin for this.
Good practice is to backup your production data before you run a migration
You can see the schema definition in db/schema.rb or db/development_structure.rb if config.active_record.schema_format = :sql

Two Special Columns

created_at and updated_at are maintained automatically by Rails and keep track of when a record was created and last updated

Migration Example


create_table "users", :force => true do |t|
  t.string :login, :email, :remember_token
  t.string :salt, :crypted_password, :limit => 40
  t.datetime :remember_token_expires_at
  t.timestamps # created_at and updated_at
end