Add a not null field in a rails table.

If you create the migration to add the new non-null column, and simply specify a default value, all records that exist will then take on the default value

class AddNotNullColumn < ActiveRecord::Migration
  def change
    add_column :table_name, :column_name, :column_type, null: false, default: 'default_value'
  end
end

This should do it all for you in one go

Create a rake task and assign default values to null columns. You can do same in migration also.

You must add migration for that because migration run only once. But rake task can be run multiple times. And don't need to worry that you run rake task or not.

Your migration should have :

def migrate(direction)
  super
  Xyz.where(:price_currency => nil).each do |xyz|
  xyz.update(:price_currency => "USD") if direction == :up
end

def change
  add_column :xyzs, :price_currency, :sting, default: "USD", null: false
end