Another approach which would be totally automatic and zero code at all, would be to set the Key field of your rates table to be a concatenated field of the fields which make the rate unique.
You can either have a scalar data entry field as the Key field and a form rule to mirror a formula field into that scalar data entry field, or if you are not doing imports, in fact you can set a formula field to be a key field via an API typed into the browser URL.
Surprisingly, while undocumented, you may have a formula field be a Key field. The only problem I have had is when importing into the table.
