function _where($key, $value = NULL, $type = 'AND ')
	{
		if ( ! is_array($key))
		{
			$key = array($key => $value);
		}
 	 	
		foreach ($key as $k => $v)
		{
			$prefix = (count($this->ar_where) == 0) ? '' : $type;
			
			if ( ! is_null($v))
			{
				if ( ! $this->_has_operator($k))
				{
					$k .= ' =';
				}
			
				$v = ' '.$this->escape($v);
			}
			else
				$v = " = NULL";
						
			$this->ar_where[] = $prefix.$k.$v;
		}
		return $this;
	}