force a row update?

In my app, I'm comparing the values in CreatedOn and ModifiedOn to determine if a row has been "touched".
The following code worked in 2.0.3, but causes an error in 2.1. The error is "Column name 'ModifiedOn' appears more than once in the result column list."

<code>
    x.ModifiedOn = DateTime.Now; // where x is an instance of ActiveRecord
    x.Save(userName);
</code>

My question is--what is the recommended way to force an update to a row?

Thanks,

R.L.