Fix COUNT(DISTINCT ...) for GROUP BY with ORDER BY and LIMIT

This is the fix for the regression of #29848.

In #29848, I've kept existing select list in the subquery for the count
if ORDER BY is given. But it had accidentally affect to GROUP BY
queries also. It should keep the previous behavior in that case.

Fixes #30886.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fix `COUNT(DISTINCT ...)` for `GROUP BY` with `ORDER BY` and `LIMIT`.
+
+    Fixes #30886.
+
+    *Ryuta Kamizono*
+
 *   PostgreSQL `tsrange` now preserves subsecond precision.
 
     PostgreSQL 9.1+ introduced range types, and Rails added support for using

activerecord/lib/active_record/relation/calculations.rb

@@ -216,7 +216,7 @@ module ActiveRecord
         if operation == "count"
           column_name ||= select_for_count
           if column_name == :all
-            if distinct && !(has_limit_or_offset? && order_values.any?)
+            if distinct && (group_values.any? || !(has_limit_or_offset? && order_values.any?))
               column_name = primary_key
             end
           elsif column_name =~ /\s*DISTINCT[\s(]+/i

activerecord/test/cases/calculations_test.rb

@@ -260,6 +260,10 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_equal 3, Account.joins(:firm).distinct.order(:firm_id).limit(3).offset(2).count
   end
 
+  def test_distinct_count_with_group_by_and_order_and_limit
+    assert_equal({ 6 => 2 }, Account.group(:firm_id).distinct.order("1 DESC").limit(1).count)
+  end
+
   def test_should_group_by_summed_field_having_condition
     c = Account.group(:firm_id).having("sum(credit_limit) > 50").sum(:credit_limit)
     assert_nil        c[1]