Attributes may be an Arel value which doesn't respond to aggregation?…

…, don't test them.

lib/arel/algebra/relations/operations/project.rb

@@ -14,7 +14,7 @@ def attributes
     end
 
     def externalizable?
-      attributes.any?(&:aggregation?) or relation.externalizable?
+      attributes.any? { |a| a.respond_to?(:aggregation?) && a.aggregation? } || relation.externalizable?
     end
   end
 end