Fix count on a separate connection (fixes #18359)

Previosly count and other AR calculations would convert
column_name_for_operation to sql on a default Arel::Table.engine
(AR::Base) connection. That could lead to trouble if current
model has a connection to a different adapter or Base connection
is inaccessible.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -392,7 +392,7 @@ def lookup_cast_type(sql_type) # :nodoc:
       end
 
       def column_name_for_operation(operation, node) # :nodoc:
-        node.to_sql
+        visitor.accept(node, collector).value
       end
 
       protected

activerecord/test/cases/multiple_db_test.rb

@@ -93,6 +93,13 @@ def test_connection
     assert_not_equal Entrant.arel_engine.connection, Course.arel_engine.connection
   end
 
+  def test_count_on_custom_connection
+    ActiveRecord::Base.remove_connection
+    assert_equal 1, College.count
+  ensure
+    ActiveRecord::Base.establish_connection :arunit
+  end
+
   unless in_memory_db?
     def test_associations_should_work_when_model_has_no_connection
       begin