Question

So I have a social-bookmarking-esque setup here. 4 tables: posts, users, comments, and votes. The homepage simply points to the posts model’s index page and runs a loop over the Post.all array and displays some basic info: title, upvotes – downvotes (I call ti the post’s “score”).

Now what I need to do it order these posts in (i’m guessing) the model by their score divided by their relative creation time (how many seconds since page load was this post created?). Rails gives me a created_at column in my posts object and I have a table of votes which looks like this:

Posts model

class Post < ActiveRecord::Base
  attr_accessible  :id, :title, :url, :user_id, :comment_count, :agreement
  has_many :votes
  has_many :comments
  belongs_to :user
end

Votes model

class Vote < ActiveRecord::Base
  attr_accessible :direction, :post_id, :vote_type, :user_id
  belongs_to :user
  belongs_to :post
  belongs_to :comment
end

IDs in the post model correspond with post_id’s in the vote model.

:direction is a boolean. 0 associated with up voting and 1 associated with downvoting. Post_ID is the ID of the post the vote was cast on.

How should I order the posts so that their position on the homepage is determined by both their creation time AND their up votes – down votes?

def index
    @posts = Post.join(:votes).order(<???>)
end

ETA: @shweta is on the right track with joining the posts and votes tables. The select portion of his answer does not work, however.

Answer 1

Disclaimer: I don’t know ruby or rails, only SQL.

shweta’s answer seems really close so I will attempt to finish it with some SQL hack and slash:

order by upvotes and then created_at

@posts = Post.joins(:votes).select("posts.id,sum(if(direction = 0, 1, -1)) as score,posts.created_at,OTHER_ATTRS_YOU_NEED").group("posts.id").order("score DESC,posts.created_at")

Also, to include posts with zero votes you will need to do a left join:

@posts = Post.joins("LEFT JOIN votes ON posts.id = votes.post_id").select("posts.id,sum(if(direction = 0, 1, -1)) as score,posts.created_at,OTHER_ATTRS_YOU_NEED").group("posts.id").order("score DESC,posts.created_at")

Update: to fix the ordering for un-voted posts:

@posts = Post.joins("LEFT JOIN votes ON posts.id = votes.post_id").select("posts.id,sum(if(direction = 0, 1, if(direction is null, 0, -1))) as score,posts.created_at,OTHER_ATTRS_YOU_NEED").group("posts.id").order("score DESC,posts.created_at")

Update 2: improved ranking

If you exclude older items you might end up with an empty list (eg on quiet days), which is probably not what you want. You might be better off adding another column for ranking, something like this:

@posts = Post.joins("LEFT JOIN votes ON posts.id = votes.post_id").select(
    "posts.id," +
    "sum(if(direction = 0, 1, if(direction is null, 0, -1))) as score," +
    "(sum(if(direction = 0, 1, if(direction is null, 0, -1))) * " +
    " if(unix_timestamp() - unix_timestamp(posts.created_at) < 7200, 3, 1)) as rank," +
    "posts.created_at,OTHER_ATTRS_YOU_NEED"
).group("posts.id").order("rank DESC,posts.created_at")

For ranking purposes, this triples the score of posts less than 2 hours old.