Obie Fernandez

Adding a Counter Cache to Existing Records

Adding a counter cache to a has_many/belongs_to ActiveRecord relationship after the underlying records have been around awhile is one of those tasks that tends to trip up Rails newbies.

Here's an example migration showing the necessary steps.

1. Add the counter_cache column as an integer. (If following normal rails convention, name the column with the plural name of the has_many association followed by _count)
2. Make sure that the column is set to disallow null values and defaults to 0.
3. Reset the counters for existing records. The reset_counters method is not smart enough to take an array of ids, so you have to call it for each record you want updated.

class AddCommentsCountToNominations < ActiveRecord::Migration
  def self.up
    add_column :nominations, :comments_count, :integer, null:false, default: 0
    # reset cached counts for nominations with comments only
    ids = Set.new
    Comment.all.each {|c| ids << c.nomination_id}
    ids.each do |nomination_id|
      Nomination.reset_counters(nomination_id, :comments)
    end
  end
  def self.down
    remove_column :nominations, :comments_count
  end
end

The :counter_cache option is covered in detail on pages 195-196 of my book The Rails 3 Way, including some interesting gotchas.

Deep Integration of Ruby with Semantic Web Ontologies

Download Deep Integration of Ruby with Semantic Web Ontologies (PDF, 225k)

My presentation file needed a new home since the primary link that comes up in Google when you search for it is a ThoughtWorks server that no longer exists.

I'm also hosting a related whitepaper by Dominic Mainz from 2009

Deep Integration of Semantic Web Using Ruby (PDF, 7.7mb)

Incidentally, it's been about 6 years since I first took interest in this subject and I still think there's room for significant innovation here. Maybe I've read too much Neal Stephenson, but I can imagine people moonlighting as domain experts and maintaining a global commons ontological library that can be imported to form the foundation of your application's domain model just as easily as we import ruby gems today. A similar intellectual energy drove Wikipedia -- why not make it work for software models?

where are un-ordered tools?

While digging up some stuff on my old blog I found this post: The new dynamics of strategy: Sense-making in a complex and complicated world that I published back in December of 2004. Almost 7 years later and I'm not so sure that we've made progress with real-world tools leveraging this research.

The authors coin the term un-order in much the same sense as undead, to refer to "emergent order". Not an absence of order, but instead the patterns that emerge from situations which undergo continual change, subject to chaotic influences. This is of particular interest to the agile community and particularly XPers, whose motto isembrace change, which explains why it seems to be getting a lot of attention from agile bloggers. I see a window of opportunity opening up for a slew of open-source projects that build on this information. While reading Wisdom of Crowds, I got the strong impression that a far-flung organization (such as the the consulting company that I work for), will benefit by moving agressively on these knowledge management advances. For instance, devise a polling tool with differing modes that match the various identities that we live on a day-to-day basis, namely: consultant, coach, manager, member of our client team, member of the parent company. Within each of these modes, features that allow referendums and continual feedback for all levels: personal, team and organization.

Maybe Asana is a step in the right direction? I've been using it with my new startup as our project management tool for everything that doesn't fit into Pivotal Tracker. Liking it so far.