Grouped query performance of total_count, take 2

Improved performance for total_count on grouped ActiveRecord::Relation.

This re-implements kaminari#979 using Arel to avoid issues kaminari#1012 and kaminari#1015

kaminari-activerecord/lib/kaminari/activerecord/active_record_relation_methods.rb

@@ -32,15 +32,13 @@ def total_count(column_name = :all, _options = nil) #:nodoc:
 
       c = c.limit(max_pages * limit_value) if max_pages && max_pages.respond_to?(:*)
 
-      # .group returns an OrderedHash that responds to #count
-      c = c.count(column_name)
-      @total_count = if c.is_a?(Hash) || c.is_a?(ActiveSupport::OrderedHash)
-                       c.count
-                     elsif c.respond_to? :count
-                       c.count(column_name)
-                     else
-                       c
-                     end
+      # Handle grouping with a subquery
+      @total_count = if c.group_values.any?
+        sq = c.except(:select).select("1 AS record").arel.as("subquery")
+        c.model.connection.select_value Arel::SelectManager.new.from(sq).project(sq[:record].count)
+      else
+        c.count(column_name)
+      end
     end
 
     # Turn this Relation to a "without count mode" Relation.

kaminari-core/test/models/active_record/active_record_relation_methods_test.rb

@@ -105,13 +105,13 @@ class ActiveRecordRelationMethodsTest < ActiveSupport::TestCase
       end
 
       test 'calculating STI total_count with GROUP BY clause' do
-        {
-          'Fenton'   => Dog,
-          'Bob'      => Dog,
-          'Garfield' => Cat,
-          'Bob'      => Cat,
-          'Caine'    => Insect
-        }.each { |name, type| type.create!(name: name) }
+        [
+          ['Fenton',   Dog],
+          ['Bob',      Dog],
+          ['Garfield', Cat],
+          ['Bob',      Cat],
+          ['Caine', Insect]
+        ].each { |name, type| type.create!(name: name) }
 
         assert_equal 3, Mammal.group(:name).page(1).total_count
       end