So I was coding peacefully when our product owner was doing some tests on the new features we implemented over that past two weeks. He pulls, bundles, runs the server and goes to the index route only to find the cheerful an empty page. “Where are my previous records?” he might have thought, “I didn’t drop the db”. He’s of a good technical expierence so he fires up Rails console to inspect this issue; one command, press Enter, and Voila, the documents are there!

He decides to call me to tell me that I’m a lousy developer that doesn’t deserve his salary and to fix this issue ASAP so he can deserve his salary too :).

So what do we have in here? A model named Post (not a real example, but the point is the same) with a pretty straight-forward lines:

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class Post
  include Mongoid::Document

  field :title
  field :content
  field :published, type: Boolean, default: true
  field :visits_count

  scope :most_visited, Proc.new { where(published: true).desc(:visits_count) }
end

The controller is no different too:

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class PostsController < ApplicationController
  def index
    @posts = Post.most_visited
  end
end

So what’s the problem? I don’t trust anyone by default when it comes to code, so I switch to the console and see for my self:

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1.9.3p327 :001 > Post.most_visited.to_a
=> []
1.9.3p327 :002 > Post.all.to_a
=> [#<Post _id: 50f81a92c12d4d6b35000002, _type: nil, title: 'blah', content: 'another blah', published: true, visits_count: 5>, #<Post _id: 50f81a92c12d4d6b35000003, _type: nil, title: 'meh', content: 'another meh', published: true, visits_count: 3>]

OK, he wasn’t lying about it, but what the X? The scope is only using one condition (published: true) and Post.all says that all records has published set to true. After debugging and checking everything that could cause such weirdness, he decided that it’s a waste of time to continue and dropping database and starting from scratch would be the solution for now. As a shot in the dark, I decided to check Mongo itself, maybe it can tell us what on Earth is going on.

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$ mongo
> use blog_development
> db.posts.findOne()
{
        "_id" : ObjectId("50f81a92c12d4d6b35000003"),
        "title" : "blah",
        "content" : "another blah",
        "visits_count" : 5
}

See the issue? There’s no published propery for the document! Then what are those documents we got from Rails console?

Turns out the problem was that I added published field to the model after those documents were already persisted, Mongoid was just filling in published because it got it in the model, set it to true because I told it to. We were telling Mongo to filter by published but it doesn’t find any published properties on the documents so it returns nothing. Pretty simple but Mongoid made us take it for granted that published was there, Damn you Mongoid, this isn’t the first time!

The rest is easy, calling save on the documents made everything work just fine (a quick-n-dirty solution, running update_all should be the way), but we learned a valuable lesson, you don’t mess with fields with default value without making a Rake task to populate them, or you run in circles while the solution was just there, hidden in plain sight.

PS: We’re good friends, me and the product owner, I’m just being sarcastic here :D.