rails5-depot/db/migrate/20161116091835_combine_items_in_cart.rb

class CombineItemsInCart < ActiveRecord::Migration[5.0]
  def up
    Cart.all.each do |cart|
      sums = cart.line_items.group(:product_id).sum(:quantity)

      sums.each do |product_id, quantity|
        if quantity > 1
          cart.line_items.where(product_id: product_id).delete_all

          item = cart.line_items.build(product_id: product_id)
          item.quantity = quantity
          item.save!
        end
      end
    end
  end

  def down
    LineItem.where('quantity > 1').each do |li|
      li.quantity.times do
        # item = Cart.find_by(:cart_id li.cart_id).line_items.build(product_id: li.product_id)
        # item.save!
        LineItem.create(cart_id: li.cart_id,
                        product_id: li.product_id,
                        quantity: 1)
      end
      li.destroy
    end
  end
end