ruby-on-rails – 从另一个模型的数组中检索唯一的关联模型

为另一个模型的子集找到多个唯一关联模型的推荐方法是什么？例如,对于一部分用户,确定他们所青睐的独特艺术家模型.

一种方法是从数据库中获取用户,然后迭代它们所有查询收藏夹并构建一个独特的数组,但这似乎效率低且速度慢.

class User < ActiveRecord::Base
  has_many :favorites
end

class Artist < ActiveRecord::Base
  has_many :favorites
end

class Favorite < ActiveRecord::Base
  belongs_to :user
  belongs_to :artist
end

@users = User.find_by_age(26)
# then determine unique favorited artists for this subset of users.

has_many关联有一个名为uniq的选项用于此要求：

class User < ActiveRecord::Base
  has_many :favorites
  has_many :artists, :through => :favorites, :uniq => true
end

class Artist < ActiveRecord::Base
  has_many :favorites
  has_many :users, :through => :favorites, :uniq => true
end

class Favorite < ActiveRecord::Base
  belongs_to :user
  belongs_to :artist
end

用法：

# if you are expecting an array of users, then use find_all instead of find_
@users = User.find_all_by_age(26, :include => :artists)
@users.each do |user|
  user.artists # unique artists
end

编辑1

我已根据用户的评论更新了答案.

解决方案1-：组

Artist.all(:joins => :users, :group => :id, 
  :conditions => ["users.age = ?", 26])

解决方案2-选择DISTINCT

Artist.all(:joins => :users, :select => "DISTINCT artists.*", 
  :conditions => ["users.age = ?", 26]))