反向的多态关联

Question

Let's say that I have two models - one called and one other called . I then have a third model - - that is polymorphically associated to to each of these models.

I can then easily do and to find comments assosciated to records of these models. All easy so far.

But what if I want to go the other way and I want to find ALL and that have been commented on and display these in a list sorted on the date that the was made? Is this possible?

If it helps I'm working on Rails 3 beta.

Answer 1

Try this:

class Post < ActiveRecord::Base
  has_many :comments, :as => :commentable
  named_scope :with_comments, :joins => :comments, 
                              :order => "comments.created_at DESC"
end

class Video < ActiveRecord::Base
  has_many :comments, :as => :commentable
  named_scope :with_comments, :joins => :comments, 
                              :order => "comments.created_at DESC"
end

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
end

Now you can run these commands:

Post.with_comments
Video.with_comments

Edit It looks like you want a single list with Videos and Posts. This is quite tricky, but doable. For every page you will have to execute 3 queries.

def commented_videos_posts(page = 1, page_size = 30)
  # query 1: get the lastest comments for posts and videos
  comments = Comment.find_all_by_commentable_type(["Post", "Video"], 
               :select => "commentable_type, commentable_id, 
                              MAX(created_at) AS created_at",
               :group => "commentable_type, commentable_id"
               :order => "created_at DESC", 
               :limit => page_size, :offset => (page-1)*page_size)

  # get the video and post ids
  post_ids, video_ids = [], []
  comments.each do |c|
    post_ids << c.commentable_id if c.commentable_type == "Post"
    video_ids << c.commentable_id if c.commentable_type == "Video"
  end

  posts, videos = {}, {}

  # query 2: get posts
  Post.all(post_ids).each{|p| posts[p.id] = p }
  # query 3: get videos
  Video.all(video_ids).each{|v| videos[v.id] = v }

  # form one list of videos and posts
  comments.collect do |c| 
    c.commentable_type == "Post" ? posts[c.commentable_id] :
                                   videos[c.commentable_id]
  end
end

Answer 2

probably not the best way, but this worked for me:

#get all posts and videos
posts_and_videos = Comment.all.collect{ |c| c.commentable }

#order by last comment date
posts_and_videos_ordered = posts_and_videos.sort{ |x,y| x.comment_date <=> y.comment_date }

hope this works for you too.

EDIT

i'm also assuming you're using has_many :comments, :as => :commentable and belongs_to :commentable, :polymorphic => true like KandadaBoggu suggested.

EDIT #2

actually, i think i made a mistake and the code above won't work... try this:

(Comment.find(:all, :order => "comment_date")).collect{ |x| x.commentable }