Fix TypeError: no implicit conversion of Arel::Attributes::Attribute into String properly

This reverts 27c6c07 since `arel_attr.to_s` is not right way to avoid
the type error.

That to_s returns `"#<struct Arel::Attributes::Attribute ...>"`, there
is no reason to match the regex to the inspect form.

And also, the regex path is not covered by our test cases. I've tweaked
the regex for redundant part and added assertions for the regex path.

2 files changed, 3 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 0fa5ba2e50..3ef6e7928f 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -245,7 +245,7 @@ module ActiveRecord
             if distinct && (group_values.any? || select_values.empty? && order_values.empty?)
               column_name = primary_key
             end
-          elsif /\s*DISTINCT[\s(]+/i.match?(column_name.to_s)
+          elsif column_name.is_a?(::String) && /\bDISTINCT[\s(]/i.match?(column_name)
             distinct = nil
           end
         end
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index 4d3db912c5..5b5202d167 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -432,6 +432,8 @@ class CalculationsTest < ActiveRecord::TestCase
   def test_should_count_selected_field_with_include
     assert_equal 6, Account.includes(:firm).distinct.count
     assert_equal 4, Account.includes(:firm).distinct.select(:credit_limit).count
+    assert_equal 4, Account.includes(:firm).distinct.count("DISTINCT credit_limit")
+    assert_equal 4, Account.includes(:firm).distinct.count("DISTINCT(credit_limit)")
   end
 
   def test_should_not_perform_joined_include_by_default