I'm attempting to create an XML builder file that tells a user to know exactly what fields failed validation in the output. I also want to display their input back to them, so that requires me figuring out which fields failed validation. Meaning if someone fails on creating a new user resource, I want to display XML that's meaningful (Besides a meaningful HTTP status number) such as:

<errors>
    <user>
        <email>bad@email: Invalid email format</email>
    </user>
<errors>

The above is tough to do in an XML builder file without knowing what field failed. And if I just iterate over error messages, I won't know how to prob my @user object to get the value that the user supplied.