#23 Counter Cache Column

Apr 25, 2007 | 7 minutes | Performance, Active Record

If you need to display the record count for a has_many association, you can improve performance by caching that number in a column.

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Update (3/29/08)

The migration code shown in this screencast does not work in Rails 2.0. Instead please use the code below. Special thanks to Josh Owens for this blog post describing the problem and solution.

          migrations/006_add_tasks_count.rb
                
        def self.up
  add_column :projects, :tasks_count, :integer, :default => 0

  Project.reset_column_information
  Project.find(:all).each do |p|
    Project.update_counters p.id, :tasks_count => p.tasks.length
  end
end

def self.down
  remove_column :projects, :tasks_count
end
        
def self.up
  add_column :projects, :tasks_count, :integer, :default => 0

  Project.reset_column_information
  Project.find(:all).each do |p|
    Project.update_counters p.id, :tasks_count => p.tasks.length
  end
end

def self.down
  remove_column :projects, :tasks_count
end

          models/task.rb
                
        belongs_to :project, :counter_cache => true
        
belongs_to :project, :counter_cache => true

          projects/index.rhtml
                
        <%= pluralize project.tasks.size, 'task' %>
        
<%= pluralize project.tasks.size, 'task' %>

You can also find the original code here which should work for Rails versions before 2.0.