Fix distinct.count when using select with multiple arel expressions

Author: fatkodima

activerecord/lib/active_record/relation/calculations.rb

@@ -635,8 +635,12 @@ def select_for_count
 
           adapter_class = model.adapter_class
           select_values.map do |field|
-            column = arel_column(field.to_s) do |attr_name|
-              Arel.sql(attr_name)
+            column = if Arel.arel_node?(field)
+              field
+            else
+              arel_column(field.to_s) do |attr_name|
+                Arel.sql(attr_name)
+              end
             end
 
             if column.is_a?(Arel::Nodes::SqlLiteral)

activerecord/test/cases/calculations_test.rb

@@ -623,6 +623,13 @@ def test_count_selected_arel_attribute
     assert_equal 4, Account.distinct.select(Account.arel_table[:firm_id]).count
   end
 
+  def test_count_selected_arel_attributes
+    # Only MySQL supports COUNT with multiple columns, and only with DISTINCT.
+    skip unless current_adapter?(:Mysql2Adapter, :TrilogyAdapter)
+
+    assert_equal 5, Account.distinct.select(Account.arel_table[:id], Account.arel_table[:firm_id]).count
+  end
+
   def test_count_with_column_parameter
     assert_equal 5, Account.count(:firm_id)
   end