ActiveRecord ::关系返回nil为＆＃39;产品＆＃39;表格信息

Question

当我尝试使用db：seed时，我在rails控制台中得到了上面的错误：

 => #<ActiveRecord::Relation [#<Product id: 16, name: nil, price: nil, active: true, description: nil, image_url: nil, professor: nil, created_at: "2015-04-24 06:59:58", updated_at: "2015-04-24 06:59:58">, #<Product id: 17, name: nil, price: nil, active: true, description: nil, image_url: nil, professor: nil, created_at: "2015-04-24 06:59:58", updated_at: "2015-04-24 06:59:58">, #<Product id: 18, name: nil, price: nil, active: true, description: nil, image_url: nil, professor: nil, created_at: "2015-04-24 06:59:58", updated_at: "2015-04-24 06:59:58">]>  

这是我的种子.rb：

Product.delete_all
Product.create( name: 'Montages simples ikebana en chocolat',
           description:
           %{
           Créez des montages commerciaux simple et rapide pour mettre en valeur votre vitrine.
           },
           image_url: 'IMG_4543.JPG',
           price: 9.00,
           professor: 'avec Jérémy Fages', active: true)
Product.create( name: 'Example2',
           description:
           %{
           Miam miam.
           },
           image_url: 'miam.jpg',
           price: 'gratuit',
           professor: 'avec Jérémy Fages', active: true)

这是我的*********** _ create_products.rb：

class CreateProducts < ActiveRecord::Migration
def change
    create_table :products do |t|
        t.string :name
        t.decimal :price, precision: 12, scale: 3
        t.boolean :active
        t.text :description
        t.string :image_url
        t.text :professor

        t.timestamps
    end
end
end

和我的Model / products.rb：

class Product < ActiveRecord::Base
  has_many :order_items

  default_scope { where(active: true) }
end

我尝试了命令rake db：drop rake db：create rake db：migrate ...

这是我的Model / Order_item.rb：

class OrderItem < ActiveRecord::Base
  belongs_to :product
   belongs_to :order

  validates :quantity, presence: true, numericality: { only_integer: true, greater_than: 0 }
  validate :product_present
  validate :order_present

  before_save :finalize

  def unit_price
    if persisted?
      self[:unit_price]
    else
       product.price
   end
  end

  def total_price
    unit_price * quantity
  end

private
  def product_present
    if product.nil?
      errors.add(:product, "is not valid or is not active.")
    end
  end

  def order_present
    if order.nil?
      errors.add(:order, "is not a valid order.")
    end
  end

  def finalize
     self[:unit_price] = unit_price
    self[:total_price] = quantity * self[:unit_price]
  end
end

关于代码，它在使用sqlite3的应用程序上运行。问题出现在我使用mysql2的其他应用程序上。