Question

I’m having an issue with ordering in Mongoid (2.0.0.beta.17 and 2.0.2)/MongoDB. Perhaps I’m just not experienced enough with MongoDB, and some of you can help me understand what I’m doing wrong?

The user-level symptoms are:

• Queries not sorting correctly by date or ID
• User posts are not appearing when they seem like they should be (this is a Twitter-like social networking site for gardeners, and the most recent posts are sometimes not showing up)

---

irb(main):024:0> Update.all.size 

=> 551

Update.ordered.size     # (see below for definition) 

=> 490

irb(main):010:0> Update.all.select{|u| u.created_at.nil?} 

=> []

When I go into the mongo shell, and do:

var cursor = db.updates.find({}, {'_id': 1}).limit(600); 
while (cursor.hasNext()) printjson(cursor.next()); 

I get 454 lines returned.

var cursor = db.updates.find({}, {'_id': 1}).sort({created_at : 1}).limit(600); 
while (cursor.hasNext()) printjson(cursor.next());   

Also returns 454 lines.

db.updates.find({}).sort({created_at: -1}).limit(1); 

returns an update from February 23rd. But I have updates from yesterday in MongoDB.
Any ideas?

---

My model is:

class Update 
  include Mongoid::Document 
  include Mongoid::Timestamps 
  include Paperclip 
  field :body 
  ... 
  index [[ :created_at, Mongo::DESCENDING ]] 
  ... 
  named_scope :ordered, :order_by => ([[:created_at, :desc]]) 
  ... 
end 

Answer 1

I didn’t figure out WHY, but doing the following seems to fix the problem:

named_scope :ordered, :order_by => ([[:created_at, :desc], [:_id, :desc]])

It seems the secondary ordering helps.

irb(main):022:0> update = Update.order_by([[:created_at, :desc], [:_id, :desc]])

irb(main):025:0> update.count
=> 556
irb(main):026:0> update.first.created_at
=> Sat May 28 02:53:33 -0700 2011
irb(main):027:0> update.second.created_at
=> Fri May 27 17:55:38 -0700 2011
irb(main):028:0> update.first.created_at.class
=> Time
irb(main):029:0> update.second.created_at.class
=> Time

The first of these was being left out of the original query.