Rails Arel选择不同的列

解决方案1

这不使用AREL,但使用Rails 2.x语法:

Topic.all(:select => "topics.*, C.id AS last_comment_id, 
                       C.created_at AS last_comment_at",
          :joins => "JOINS (
             SELECT DISTINCT A.id, A.topic_id, B.created_at
             FROM   messages A,
             (
               SELECT   topic_id, max(created_at) AS created_at
               FROM     comments
               GROUP BY topic_id
               ORDER BY created_at
               LIMIT 5
             ) B
             WHERE  A.user_id    = B.user_id AND 
                    A.created_at = B.created_at
           ) AS C ON topics.id = C.topic_id
          "
).each do |topic|
  p "topic id: #{topic.id}"
  p "last comment id: #{topic.last_comment_id}"
  p "last comment at: #{topic.last_comment_at}"
end

确保索引 created_at 和 topic_id 列中 comments 表。

解决方案2

添加一个 last_comment_id 你的专栏 Topic 模型。更新 拉斯特 创建注释后。这种方法比使用复杂的SQL来确定最后一条注释快得多。

例如:

class Topic < ActiveRecord::Base
  has_many :comments
  belongs_to :last_comment, :class_name => "Comment"
  scope :hot, joins(:last_comment).order("comments.created_at DESC").limit(5)
end

class  Comment
  belongs_to :topic

  after_create :update_topic

  def update_topic
    topic.last_comment = self
    topic.save
    # OR better still
    # topic.update_attribute(:last_comment_id, id)
  end
end

这比运行复杂的SQL查询来确定热门主题要有效得多。

在大多数SQL实现中,这并不是那么优雅。一种方法是首先获取按主题ID分组的五个最近评论的列表,然后通过用in子句进行子选择来获取comments.created。

我对阿雷尔很陌生,但像这样的事情可以奏效。

recent_unique_comments = Comment.group(c[:topic_id]) \
                                .order('comments.created_at DESC') \
                                .limit(5) \
                                .project(comments[:topic_id]
recent_topics = Topic.where(t[:topic_id].in(recent_unique_comments))

# Another experiment (there has to be another way...)

recent_comments = Comment.join(Topic) \
                         .on(Comment[:topic_id].eq(Topic[:topic_id])) \ 
                         .where(t[:topic_id].in(recent_unique_comments)) \
                         .order('comments.topic_id, comments.created_at DESC') \
                         .group_by(&:topic_id).to_a.map{|hsh| hsh[1][0]}

为了实现这一点,您需要使用 GROUP BY 获取每个主题的最新评论。然后您可以订购这个范围 created_at 获取最近对主题的评论。

下面的代码适用于我使用sqlite

class Comment < ActiveRecord::Base

  belongs_to :topic

  scope :recent, order("comments.created_at DESC")
  scope :latest_by_topic, group("comments.topic_id").order("comments.created_at DESC")
end


class Topic < ActiveRecord::Base
  has_many :comments

  scope :hot, joins(:comments) & Comment.latest_by_topic & limit(5)
end

我使用以下seeds.rb生成测试数据

(1..10).each do |t|
  topic = Topic.new
  (1..10).each do |c|
    topic.comments.build(:subject => "Comment #{c} for topic #{t}")
  end
  topic.save
end

下面是测试结果

ruby-1.9.2-p0 > Topic.hot.map(&:id)
 => [10, 9, 8, 7, 6] 
ruby-1.9.2-p0 > Topic.first.comments.create(:subject => 'Topic 1 - New comment')
 => #<Comment id: 101, subject: "Topic 1 - New comment", topic_id: 1, content: nil, created_at: "2010-08-26 10:53:34", updated_at: "2010-08-26 10:53:34"> 
ruby-1.9.2-p0 > Topic.hot.map(&:id)
 => [1, 10, 9, 8, 7] 
ruby-1.9.2-p0 > 

为sqlite生成的SQL(重新格式化)非常简单,我希望AREL为其他引擎呈现不同的SQL,因为在许多DB引擎中,这肯定会失败,因为主题中的列不在“分组依据列表”中。如果这确实存在问题,那么您可以通过将所选列限制为comments.topic\u id来克服它。

puts Topic.hot.to_sql
SELECT     "topics".* 
FROM       "topics" 
INNER JOIN "comments" ON "comments"."topic_id" = "topics"."id" 
GROUP BY  comments.topic_id 
ORDER BY  comments.created_at DESC LIMIT 5

因为这个问题是关于阿雷尔的,所以我想我应该添加这个,因为Rails3.2.1添加了 uniq 查询方法:

如果添加 .uniq 在阿雷尔的作品中增加了 DISTINCT 到 select 语句。

例如 Topic.hot.uniq

也在范围内工作:

例如 scope :hot, joins(:comments).order("comments.created_at DESC").limit(5).uniq

所以我假设

scope :hot, joins(:comments) & Comment.recent & limit(5) & uniq

也可能有效。

见 http://apidock.com/rails/ActiveRecord/QueryMethods/uniq