一、先上代码

class Variant
    validate :check_price

      # Ensures a new variant takes the product master price when price is not supplied
      def check_price
        if price.nil? && Config[:require_master_price]
          raise 'No master variant found to infer price' unless (product && product.master)
          raise 'Must supply price for variant or master.price for product.' if self == product.master
          self.price = product.master.price
        end
        if currency.nil?
          self.currency = Config[:currency]
        end
      end

class LineItem
    belongs_to :variant, inverse_of: :line_items

    before_validation :copy_price

    def copy_price
      if variant
        update_price if price.nil?
        self.cost_price = variant.cost_price if cost_price.nil?
        self.currency = variant.currency if currency.nil?
      end
    end

二、数据库

variant 数据库列中没有 currency 列

+----+-----------+--------+--------+-------+-------+------------+-----------+------------+------------+------------+----------+-------------+------------+-------------+------------+
| id | sku       | weight | height | width | depth | deleted_at | is_master | product_id | cost_price | cost_cu... | position | track_in... | tax_cat... | updated_at  | stock_i... |
+----+-----------+--------+--------+-------+-------+------------+-----------+------------+------------+------------+----------+-------------+------------+-------------+------------+
| 1  | R00011 | 0.0    |        |       |       |            | true      | 1          | 17.0       | USD        | 2        | true        |            | 2015-07-... | 1          |
+----+-----------+--------+--------+-------+-------+------------+-----------+------------+------------+------------+----------+-------------+------------+-------------+------------+
1 row in set

三、Active Record 数据验证

以下方法会做数据验证，如果验证失败就不会把对象存入数据库： create create! save save! update update!

四、问题

问题：variant 数据库列中没有 currency 列，所以不能持久化，但是 LineItem 中能使用 variant.currency，难道每次 variant 数据出库会执行 validate :check_price 吗？ validate :check_price 在数据创建和读取都会调用吗？read 方法也会执行数据验证吗？