Counter Caching in Rails

Finding Number Of Answers For A Question In Rails 🔗

Earlier this week I was given a pretty straightforward user story of figuring out how many Answers a given Question had received.

Wrote my acceptance tests and then was surprised at how complicated this was to actually implement. After devouring the Active Record documentation in hopes of finding a method that would leverage the two models associations I ended up bootstrapping a method to get it to work:

# in Question Model:
def self.unanswered
  unanswered = []
  Question.all.each do |q|
    if q.answers.count == 0
      unanswered << q
    end
  end
  unanswered
end

I knew that I hated this method and it had N + 1 written all over it. After discussing solutions with one of our Senior team members we ended up writing this gnarly SQL statement to find the id's of all the questions with no answers associated to them.

Finally, we decided it would be best to just implement a Counter Cache on the Question model. Here’s how to do that:

class AddAnswerCountToQuestions < ActiveRecord::Migration
  class Question < ActiveRecord::Base
    has_many :answers
  end

  class Answer < ActiveRecord::Base
    belongs_to :question, counter_cache: true
  end

  def up
    add_column :questions, :answers_count, :integer, null: false, default: 0

    Question.pluck(:id).each do |id|
      Question.reset_counters(id, :answers)
    end
  end

  def down
    remove_column :questions, :answers_count
  end
end

What is this migration doing? 🔗

• Adding a answers_count column to the Questions model so that everytime an answer gets created for a question that will go up by 1. It also sets a default value of 0 since when a Question gets created it has no answers yet.
• Lines 13-15 then go back through all prior Questions created and reset the new answers_count column to 0 so we don’t have any issues with legacy questions.
• The down method is set up for if we have to rollback changes ever.
• Finally, the reason we have the two classes inside our migration is so that if we ever change the name of Question, it won’t ruin the migration and throw errors.

Changing The Real Models 🔗

In our migration we adjusted the belongs_to method to accept the counter_cache: true but that was just for our dummmy models in the migration. Now we need to change our actual models like this:

class Answer < ActiveRecord::Base
  belongs_to :user
  belongs_to :question, counter_cache: true
end

class Question < ActiveRecord::Base
  has_many :answers
end

The last thing to finish the original user story is to refactor that original method to implement the counter cache. Here is how I implemented it:

def self.unanswered
  where(answers_count: 0)
end

Much, much cleaner than the original method!