How to copy other column's value as default value for a new column in rails migration?

Question

I have a model with a column price. I need to add a new_column marked_price having value of price as it's default value. Can I write this in my migration, or what could be the best way to do this?

Something like:

class AddMarkedPriceToMenuItems < ActiveRecord::Migration
  def change
    add_column :menu_items, :marked_price, :decimal, :default => :price
  end
end

Answer 1

No, the database does not allow you to do this using the DEFAULT setting on a table column.

But you can do it using an ActiveRecord callback

class MenuItem < ActiveRecord::Base
  before_create :set_market_price_default

  private

  def set_market_price_default
    self.market_price = self.price
  end
end

As for the migration itself, you can update market_price manually

def change
  add_column :menu_items, :marked_price, :decimal

  reversible do |dir|
    dir.up { MenuItem.update_all('marked_price = price') }
  end
end

Note that you might want to create a copy of the model that sits locally in the migration, so that it doesn't go out of sync in the future.