从另一个模型的数组中检索唯一的关联模型

Question

为另一个模型的子集查找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，确定他们喜欢的独特艺术家模型。

一种方法是从数据库中获取用户，然后遍历所有用户以查找收藏夹并构建唯一数组，但这似乎效率低下且速度慢。

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.

Answer 1

has_many 关联有一个名为 uniq 的选项来满足此要求：

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- :group

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

解决方案 2- SELECT DISTINCT

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