/**
  * Quotes primitive value with its primitve variable (alias.var)
  * @param mixed $v
  * @param string $pvar
  * @return true|string (error message if string)
  */
 public function quoteVar(&$v, &$pvar)
 {
     // unquote pvar
     $pvar_ = $this->_unquote($pvar);
     // split primitive var to alias and var
     list($alias, $var) = @explode('.', $pvar_);
     if (!$alias || !$var) {
         return "invalid primitive var '{$pvar}'";
     }
     // get the class for alias
     if (!($class = $this->am->getClass($alias))) {
         return "no class found for alias '{$alias}'";
     }
     // get class map
     if (!($cm = $this->proot->getClassMap($class))) {
         return "no class map for '{$class}'";
     }
     // is var 'oid'?
     if ($var == 'oid') {
         // replace var name with column name
         $pvar = $this->quoteId($alias) . '.' . $this->quoteId($cm->getOidColumn());
         return true;
     }
     // get field map (for non-oid field)
     if (!($fm = $cm->getField($var))) {
         return "no field map for '{$class}::{$var}'";
     }
     // replace var name with column name
     $pvar = $this->quoteId($alias) . '.' . $this->quoteId($fm->getColumnName());
     // quote value
     $v = $this->quote($v, $fm);
     return true;
 }