Fix `count(:all)` to correctly work `distinct` with custom SELECT list

Currently `count(:all)` with `distinct` doesn't work correctly because
SELECT list is always replaced to `*` or primary key in that case even
if having custom SELECT list.

And also, PostgreSQL has a limitation that ORDER BY expressions must
appear in select list for SELECT DISTINCT.

Therefore, we should not replace custom SELECT list when using
`count(:all)` with `distinct`.

Closes #31277.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index 55b50e4f84..5eda39a0c7 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -236,6 +236,12 @@ class CalculationsTest < ActiveRecord::TestCase
     end
   end
 
+  def test_distinct_count_all_with_custom_select_and_order
+    accounts = Account.distinct.select("credit_limit % 10").order(Arel.sql("credit_limit % 10"))
+    assert_queries(1) { assert_equal 3, accounts.count(:all) }
+    assert_queries(1) { assert_equal 3, accounts.load.size }
+  end
+
   def test_distinct_count_with_order_and_limit
     assert_equal 4, Account.distinct.order(:firm_id).limit(4).count
   end