如何通过有序关联检索有序对象？

Question

我正在使用Ruby on Rails 3.2.2。我有以下关联：

class Article < ActiveRecord::Base
  has_many :category_associations,
    :class_name => 'CategoryAssociation'

  # Note: Same as :category_associations but gets records ordered by :position.
  has_many :positioned_category_associations,
    :class_name => 'CategoryAssociation',
    :order      => :position

  has_many :categories,
    :through    => :category_associations,
    :source     => :category

  # Note: Same as :categories but gets records ordered by :position.
  has_many :positioned_categories,
    :through    => :category_associations,
    :source     => :category,
    :order      => [:category_associations => :position]
end

使用上面的代码，我可以调用@article.positioned_categories并按位置正确排序。但是，我想使用:positioned_category_associations来检索:positioned_categories ，但如果我说明以下内容：

class Article < ActiveRecord::Base
  ...

  has_many :positioned_categories,
    :through    => :positioned_category_associations,
    :source     => :category

  # instead of
  #
  # has_many :positioned_categories,
  #   :through    => :category_associations,
  #   :source     => :category,
  #   :order      => [:category_associations => :position]
end

似乎没有按预期工作：我得到不有序类别。

如何通过:positioned_category_associations检索有序类别？有可能吗？

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Bonus ：由于:category_associations和:positioned_category_associations的语句几乎相同（:order => :position除外），是否可以重构:positioned_category_associations语句那么干（不要重复自己）代码？