code: public function has($forClass, $forColumn = null)
  	{
  		$refTable = static::tableName();
  		$forTable = $forClass::tableName();
  		$refColumn = static::$idColumn;
  		$forColumn = $forColumn ?: strtolower(static::modelName())."_id";
  
  		$rows = Db::query("
  			select F.*
  			from $refTable as R, $forTable as F
  			where R.$refColumn = F.$forColumn and R.$refColumn = :id
  		", ["id" => $this->$refColumn], static::getDbName());
  
  		if ($rows === false)
  			return false;
  		if (empty($rows))
  			return null;
  		
  		// Populate model
  		$forModel = new $forClass;
  		foreach ($rows[0] as $col => $val) $forModel->$col = $forModel->decodeValue($val, $col);
  		return $forModel;
  	}
docs: One-to-one relationship
  
  @param string	$forClass	foreign model class name
  @param string	$forColumn	foreign column if it differs from className_id (ex. user_id)
  
  @return Model|bool foreign model or false if fails