I want to create a database to store information from wind export credit bonds, such as the bond code and distribution companies, and so on, but there is a "history of credit rating", is responsible for recording time bond credit rating change information, from wind to export, as shown in the form of now need to treat each rating change level and company and date of the corresponding storage, because each bond credit rating change of time and times are different, so would like to ask you have what good storage methods you need to use python implementation