AddCountCacheToPublications.rb

# coding: utf-8
class AddCountCacheToPublications < ActiveRecord::Migration
  def change
    add_column :publications, :concerns_count, :integer, null: false, default: 0
    add_column :publications, :publication_comments_count, :integer, null: false, default: 0

    Publication.reset_column_information
    Publication.all.each do |p|
      Publication.update_counters p.id, concerns_count: p.concerns.count, publication_comments_count: p.publication_comments.count
    end
  end
end

concern.rb

# coding: utf-8
class Concern < ActiveRecord::Base
  belongs_to :publication, counter_cache: true
end

index.html.haml

- @publications.each do |pub|
  = pub.concerns.size
  = pub.publication_comments.size

publication.rb

# coding: utf-8
class Publication < ActiveRecord::Base
  has_many :concerns
  has_many :publication_comments
end

publication_comment.rb

# coding: utf-8
class PublicationComment < ActiveRecord::Base
  belongs_to :publication, counter_cache: true
end

publications_controller.rb

# coding: utf-8
class PublicationsController < ApplicationController
  def index
    @publications = Publication.all
  end
end

class AddCountCacheToPublications < ActiveRecord::Migration
  def change
    add_column :publications, :concerns_count, :integer, null: false, default: 0
    add_column :publications, :publication_comments_count, :integer, null: false, default: 0

    Publication.reset_column_information
    Publication.all.each do |p|
      Publication.update_counters p.id, concerns_count: p.concerns.count, publication_comments_count: p.publication_comments.count
    end
  end
end

belongs_to :publication, counter_cache: true