active_record_group_count.rb

module ActiveRecordGroupCount
  extend ActiveSupport::Concern

  module ExtensionMethods
    def count(*args)
      scope = except(:select).select("1")
      query = "SELECT count(*) AS count_all FROM (#{scope.to_sql}) x"
      ActiveRecord::Base.connection.execute(query).first.first
    end
  end
  
  module ClassMethods
    def returns_count_sum
      scoped.extending(ExtensionMethods)
    end
  end
end

ActiveRecord::Base.send :include, ActiveRecordGroupCount

# usage:
# Model.group(:something).returns_count_sum.count
# faster Kaminari pagination:
# Model.returns_count_sum.page(1).per(10).total_pages