## app/models/app.rb
  named_scope :with_client_id, lambda { |client_id|
    return :conditions => { :client_id => client_id } if client_id
    return {}
  }

  named_scope :from_days_ago, lambda { |days|
    return :conditions => ['timestamp >= ?', days.to_i.days.ago] if days
    return {}
  }

## app/controllers/app_controller.rb
    @apps = App.
      with_client_id(params[:id]).
      from_days_ago(params[:days]).
      count(:group => 'name')