An example of adding a column as nullable, setting the default, then making it not null.

add-update-then-null.rb

class AddTimestampsToRegions < ActiveRecord::Migration[4.2]
  def up
    add_timestamps :regions, null: true
    execute "UPDATE regions SET created_at = NOW(), updated_at = NOW()"
    change_column_null :regions, :created_at, false
    change_column_null :regions, :updated_at, false
  end

  def down
    remove_column :regions, :created_at
    remove_column :regions, :updated_at
  end
end