困难的named_scope情况

class Tag < AR::Base
  named_scope :from_user, lambda { |user| 
    { :include => :taggings, :conditions => ["taggings.user_id = ?", user.id] } 
  }
end

tags = @discussion.taggings.find_by_user(@user).map(&:tag)

taggings.find_by_user_id(@user.id)

我还没有机会测试这个,但我认为它可能会奏效:

class Discussion < ActiveRecord::Base
  has_many :taggings
  has_many :tags, :through => :taggings
end

class Tagging < ActiveRecord::Base
  belongs_to :discussion
  belongs_to :tag  
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :discussions, :through => :taggings

  named_scope :by_user do
    def named(user) do
      Tagging.find_by_user_and_discussion(user, discussion).tags
    end 
  end
end

这样使用它:

tags = @discussion.tags.by_user.named(@user)