fix time typcasting on group counts in PG

2 files changed, 11 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 70d07fdd36..29f205450d 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -274,7 +274,7 @@ module ActiveRecord
         column_alias_for(field)
       }
       group_columns = group_aliases.zip(group_fields).map { |aliaz,field|
-        [aliaz, column_for(field)]
+        [aliaz, field]
       }
 
       group = group_fields
@@ -314,7 +314,10 @@ module ActiveRecord
       end
 
       Hash[calculated_data.map do |row|
-        key = group_columns.map { |aliaz, column|
+        key = group_columns.map { |aliaz, column_name|
+          column = calculated_data.column_types.fetch(aliaz) do
+            column_for(column_name)
+          end
           type_cast_calculated_value(row[aliaz], column)
         }
         key = key.first if key.size == 1
diff --git a/activerecord/test/cases/adapters/postgresql/timestamp_test.rb b/activerecord/test/cases/adapters/postgresql/timestamp_test.rb
index 630bdeec67..e6f721cc5d 100644
--- a/activerecord/test/cases/adapters/postgresql/timestamp_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/timestamp_test.rb
@@ -2,6 +2,12 @@ require 'cases/helper'
 require 'models/developer'
 
 class TimestampTest < ActiveRecord::TestCase
+  def test_group_by_date
+    keys = Topic.group("date_trunc('month', created_at)").count.keys
+    assert_operator keys.length, :>, 0
+    keys.each { |k| assert_kind_of Time, k }
+  end
+
   def test_load_infinity_and_beyond
     unless current_adapter?(:PostgreSQLAdapter)
       return skip("only tested on postgresql")