Rails ActiveRecord / ActiveRelation grouped count as integer (sum of counts for each group). Tested on Postgres This can be used for Kaminari too (speeds up total_pages and total_count considerably).

rails_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.try(:[], "count_all").to_i
    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

rails 4.0.2
kaminari 0.14.1

Line #8 raises TypeError no implicit conversion of String into Integer so I had to change

ActiveRecord::Base.connection.execute(query).first.try(:[], "count_all").to_i

to

ActiveRecord::Base.connection.execute(query).first.first

Also line #14 raises deprecation warning (change scoped to all)