Remove useless `column_alias` in `subquery_for_count`

If select clause is specified and last column has a column alias,
additional column alias causes a statement invalid.

Add test coverage for counting a single column with NULL values.

Fixes #27676, #27682, and #27705.

2 files changed, 5 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 35c670f1a1..e8aba02611 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -364,15 +364,14 @@ module ActiveRecord
       end
 
       def build_count_subquery(relation, column_name, distinct)
-        column_alias = Arel.sql("count_column")
         subquery_alias = Arel.sql("subquery_for_count")
 
-        aliased_column = aggregate_column(column_name == :all ? 1 : column_name).as(column_alias)
+        aliased_column = aggregate_column(column_name == :all ? "1" : column_name)
         relation.select_values = [aliased_column]
         subquery = relation.arel.as(subquery_alias)
 
         sm = Arel::SelectManager.new relation.engine
-        select_value = operation_over_aggregate_column(column_alias, "count", distinct)
+        select_value = operation_over_aggregate_column(Arel.star, "count", distinct)
         sm.project(select_value).from(subquery)
       end
   end
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index b29733a676..0de2fea4c2 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -166,14 +166,15 @@ class CalculationsTest < ActiveRecord::TestCase
   end
 
   def test_limit_should_apply_before_count
-    accounts = Account.limit(3).where("firm_id IS NOT NULL")
+    accounts = Account.limit(4)
 
     assert_equal 3, accounts.count(:firm_id)
     assert_equal 3, accounts.select(:firm_id).count
+    assert_equal 3, accounts.select("firm_id firm_id").count
   end
 
   def test_limit_should_apply_before_count_arel_attribute
-    accounts = Account.limit(3).where("firm_id IS NOT NULL")
+    accounts = Account.limit(4)
 
     firm_id_attribute = Account.arel_table[:firm_id]
     assert_equal 3, accounts.count(firm_id_attribute)