ActiveRecord Migrations

Background

• Great resource: Guide on Migrations
• When you discover you need a new column or table, you can modify the database
• E.g.break the “name” field into a “first name” and “last name”
• Data Migration
  • How do I move over all the user records that were already there?
• Logic Migration
  • What has to change in the server code to accomodate this change?
• Does this mean that migration is bad?
  • It’s not good nor bad: it’s unavoidable
  • It’s much more painful when you are ‘in production’
• Use Rake db:create_migratoin

Example

• Migrations usually include the standard types
• Notice change indicates that this is a reversible migration

class CreateProducts < ActiveRecord::Migration[5.0]
  def change
    create_table :products do |t|
      t.string :name
      t.text :description
    end
  end
end

Datatypes

• There are quite a number of available statements (note they are actually medthods!!):
  • add_column, add_foreign_key, add_index, add_reference, add_timestamps,
  • change_column_default (must supply a :from and :to option), change_column_null,
  • create_join_table, create_table, disable_extension,
  • drop_join_table, drop_table (must supply a block),
  • remove_column (must supply a type), remove_foreign_key (must supply a second table), remove_index, remove_reference, remove_timestamps, rename_column, rename_index, rename_table

Rails commands

• rails generate migration - creates a new migration. Look it up for how to parameters work
• rails db:migrate - applies the migrations to the existing databases
• rails db:rollback - rolls the most recent migration back (un-applies)
• rails db:reset - drop the database and set it up again

Rake/Sinatra/Activerecord commands

• rake db:create_migration - creates a new migration. Look it up for how to parameters work
• rake db:migrate - applies the migrations to the existing databases
• rake db:rollback - rolls the most recent migration back (un-applies)
• rake db:reset - drop the database and set it up again