Reference Field in Original is empty?

Hi, I have two databases: transaction and firm. I need to calculate the interest but the problem is “rate”. I’ve transaction amount & type in transaction collection and rate of interest & firm_name in firm database.

firm database contains only firm details. So, there will be one firm per user. While transactions can be more that one for the same firm. I want to use the reference field so that everytime I can get access to the rate. Is this possible?