轨道和模型验证

我想在下表中进行一些模型级验证:

create_table :audios do |t|
  t.integer :library_id, :null => false
  t.string :file, :null => false, :limit => 2048
  t.string :name, :limit => 512
  t.timestamps
end

这是否意味着我的模型(目前为止)看起来像:

class Audio < ActiveRecord::Base
  belongs_to :library
end

有

class Audio < ActiveRecord::Base
  validates_presence_of :library
  ...

或

class Audio < ActiveRecord::Base
  validates_presence_of :library_id
  ...

?