diff --git a/lib/groupdate/magic.rb b/lib/groupdate/magic.rb
index 5c7167f..550dd9a 100644
--- a/lib/groupdate/magic.rb
+++ b/lib/groupdate/magic.rb
@@ -132,11 +132,13 @@ module Groupdate
 
       count =
         begin
-         if method == :group_calc
-           Hash[ relation.select("#{relation.group_values[@group_index]} AS #{@field}", "#{args[0]} AS calculation").group("#{relation.group_values[@group_index]}").map{|record| [record.send(@field), record.calculation] } ]
-         else
-           Hash[ relation.send(method, *args, &block).map{|k, v| [multiple_groups ? k[0...@group_index] + [cast_method.call(k[@group_index])] + k[(@group_index + 1)..-1] : cast_method.call(k), v] } ]
-         end
+          calculation =
+            if method == :group_calc
+              Hash[ relation.group("#{relation.group_values[@group_index]}").pluck("#{relation.group_values[@group_index]} AS #{@field}, #{args[0]} AS calculation") ]
+            else
+              relation.send(method, *args, &block)
+            end
+          Hash[ calculation.map{|k, v| [multiple_groups ? k[0...@group_index] + [cast_method.call(k[@group_index])] + k[(@group_index + 1)..-1] : cast_method.call(k), v] } ]
         rescue NoMethodError
           raise "Be sure to install time zone support - https://github.com/ankane/groupdate#for-mysql"
         end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 8ad9331..d9dd63b 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -789,6 +789,16 @@ module TestGroupdate
     assert_equal 15, User.group_by_day(:created_at).group_calc("SUM(score) * 1.0 / COUNT(*)")[utc.parse("2014-05-01 00:00:00 UTC")]
   end
 
+  def test_group_calc_multiple_groups
+    create_user "2014-05-01 00:00:00 UTC", 10
+    create_user "2014-05-01 00:00:00 UTC", 20
+    expected = {
+      [10, utc.parse("2014-05-01 00:00:00 UTC")] => 10,
+      [20, utc.parse("2014-05-01 00:00:00 UTC")] => 20
+    }
+    assert_equal expected, User.group(:score).group_by_day(:created_at).group_calc("SUM(score) * 1.0 / COUNT(*)")
+  end
+
   # helpers
 
   def assert_format(method, expected, format, options = {})
--
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