total_count has to be retrieved from DB if loaded AR::Relation is empty

fixes #845

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

@@ -21,7 +21,7 @@ module Kaminari
         # Total count has to be 0 if loaded records are 0
         return @total_count = 0 if (current_page == 1) && @records.empty?
         # Total count is calculatable at the last page
-        return @total_count = (current_page - 1) * @_per + @records.length if defined?(@_per) && (@records.length < @_per)
+        return @total_count = (current_page - 1) * @_per + @records.length if @records.any? && defined?(@_per) && (@records.length < @_per)
       end
 
       # #count overrides the #select which could include generated columns referenced in #order, so skip #order here, where it's irrelevant to the result anyway

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

@@ -20,6 +20,20 @@ if defined? ActiveRecord
         Readership.delete_all
       end
 
+      test 'total_count on not yet loaded Relation' do
+        assert_equal 0, User.where('1 = 0').page(1).per(10).total_count
+        assert_equal 7, User.page(1).per(10).total_count
+        assert_equal 7, User.page(2).per(10).total_count
+        assert_equal 7, User.page(2).per(2).total_count
+      end
+
+      test 'total_count on loded Relation' do
+        assert_equal 0, User.where('1 = 0').page(1).per(10).load.total_count
+        assert_equal 7, User.page(1).per(10).load.total_count
+        assert_equal 7, User.page(2).per(10).load.total_count
+        assert_equal 7, User.page(2).per(2).load.total_count
+      end
+
       test 'it should reset total_count memoization when the scope is cloned' do
         assert_equal 1, User.page.tap(&:total_count).where(name: 'author').total_count
       end