ruby-on-rails – 无效小数在rails中变为0.0

class Product < ActiveRecord::Base
end

class CreateProducts < ActiveRecord::Migration
  def self.up
    create_table :products do |t|
      t.decimal :price

      t.timestamps
    end
  end

  def self.down
    drop_table :products
  end
end

但是当我在rails控制台中执行以下操作时：

ruby-1.9.2-p180 :001 > product = Product.new
 => #<Product id: nil,price: nil,created_at: nil,updated_at: nil> 
ruby-1.9.2-p180 :002 > product.price = 'a'
 => "a" 
ruby-1.9.2-p180 :003 > product.save
 => true 
ruby-1.9.2-p180 :004 > p product
#<Product id: 2,price: #<BigDecimal:39959f0,'0.0',9(9)>,created_at: "2011-05-18 02:48:10",updated_at: "2011-05-18 02:48:10">
 => #<Product id: 2,price: #<BigDecimal:3994ca8,updated_at: "2011-05-18 02:48:10">

如你所见,我写了’a’,它在数据库中保存了0.0.这是为什么？这特别令人讨厌,因为它绕过了我的验证,例如：

class Product < ActiveRecord::Base
  validates :price,:format => /d.d/
end