Question

在Rails中的典型用户 - 帖子 - 评论模型中，每个用户都可以创建帖子并且还可以创建评论，问题是如何抓住每个用户对特定帖子的最新评论。

Example:

Post A have 3 user making comment
User 1 have comment 1, 2, 3, 4, 5, 6
User 2 have comment 1, 2, 3, 4
User 3 have comment 1, 2

所以我想要的视图只是每个用户的最新评论：

Post A have 3 user making comment
User 1 latest comment that is 6
User 2 latest comment that is 4
user 3 latest comment that is 2

怎么做？

感谢

Answer 1

这样的事情：

post.comments.for_user(current_user).last

在模型中添加named_scope

class Comment 
    named_scope :for_user, lambda{ |user| {:conditions=>{:user_id => user.id}}
end

这应该可以解决问题。

如果你宁愿在rails中这样做，

messages_by_users  = post.messages.group_by(&:user)

messages_by_users.each do  |key, value|   
    messages_by_users[key] = value.last
end

Answer 2

我必须得到这种数据，通常我最终会做两个查询。在我的情况下，我有博客和他们的帖子，我想要一个最近3篇博客文章的列表，限制博客是独一无二的，我不想要来自同一博客的2个帖子。我最终做了这样的事情（MySQL）：

q = <<-EOQ
SELECT id,pub_date FROM
  (
  SELECT id,blog_id,pub_date
  FROM posts 
  ORDER BY pub_date DESC 
  LIMIT 40
  )
t
GROUP BY blog_id
ORDER BY pub_date DESC
LIMIT #{num_posts}
EOQ
post_ids = Post.connection.select_values(q)
Post.find(:all, :include => [:blog], :conditions => ["id IN (?)", post_ids], :order => "posts.pub_date DESC")

所以在你的情况下，你可能有类似的东西：

q = <<-EOQ
SELECT id FROM
  (
  SELECT id,post_id
  FROM comments 
  ORDER BY id DESC 
  LIMIT 40
  )
t
GROUP BY post_id
ORDER BY id DESC
LIMIT 10
EOQ
post_ids = Post.connection.select_values(q)
Post.find(:all, :include => [:blog], :conditions => ["id IN (?)", post_ids], :order => "posts.id DESC")

Answer 3

假设您的数据库正在为评论分配顺序ID，您可以这样做：

class Comment
  named_scope :most_recent, lambda {
     lastest_comments = Comment.maximum :id, :group => "user_id, post_id"
     { :conditions => [ "comment_id in ?", lastest_comments.map(&:last) ] }
  }
end

这为您提供了一种双查询方法，您可以通过各种方式使用它。上面的named_scope会为所有帖子上的所有用户提取最新评论。如果您的数据库很庞大，这可能会成为一个问题，但您当然可以添加条件以使其更具体。

目前，这是一种灵活的方法，可以让您执行以下操作：

Comment.most_recent.find_by_user @user   #-> the most recent comments on all posts by a user
@user.comments.most_recent               #-> same as above

Comment.most_recent.find_by_post @post   #-> the most recent comments on a single post by all users
@post.comments.most_recent               #-> same as above

Comment.most_recent.find_by_user_and_post @user, @post    #-> the specific most recent comment by a certain user on a certain post
@post.comments.most_recent.find_by_user @user             #-> you get the idea

只获取Rails中的最新评论

3 个答案: