Prevent foreign_key_for? from type casting all attributes

activerecord/lib/active_record/associations/association.rb

@@ -232,7 +232,7 @@ def invertible_for?(record)
 
         # Returns true if record contains the foreign_key
         def foreign_key_for?(record)
-          record.attributes.has_key? reflection.foreign_key
+          record.has_attribute?(reflection.foreign_key)
         end
 
         # This should be implemented to return the values of the relevant key(s) on the owner,