Conditional Global Validator (PHP)

I'd like to disable editing or deletion of a record if a certain field is already set in that record.

Looking at the manual here I think this should be possible. I'm thinking of passing in the record's ID to the validation function and then running a sql query to see if the field in question is already set for that record.

However taking the example from the manual

// Create and edit with dependent validation
Editor::inst( $db, 'table' )
    ->fields( ... )
    ->validator( function ( $editor, $action, $data ) {
        if ( $action === Editor::ACTION_CREATE || $action === Editor::ACTION_EDIT ) {
            foreach ( $data['data'] as $pkey => $values ) {
                if ( $values['country'] === 'US' && $values['location'] === 'London UK' ) {
                    return 'London UK is not in the US';
                }
            }
        }
    } )
    ->process( $_POST )
    ->json();

if I try to pass in data from the field "tablename.idfieldname" as $values['tablename.idfieldname] I get the error message "Undefined index: tablename.idfieldname" on an edit attempt.

I think this is because the value "idfieldname" isn't being edited and so isn't being passed in the data. Thus it is undefined. That's if I'm understanding this correctly.

Is there a way (there must be) to extract the the ID of the row being edited?

Many thanks in advance to anyone with the answer!