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	public function fetchById($margin_id, $forupdate = false)
	{
		if(!is_numeric($margin_id) OR $margin_id <= 0){
			throw new Exception("Invalid margin id", BTCChina_ErrorCode::INVALID_PARAMETER);
		}
		$margin_id = (int)$margin_id;

		$select = $this->_db->select()
			->from("margin")
			->where("margin_id = ?", $margin_id)
			->forUpdate($forupdate);
		return $this->_db->fetchRow($select);
	}