I want to have a common unique transaction number reference

I want to have a common unique transaction number reference for different objects. For example. I have an object for fish and another object for fruits. They are of different transaction type. one can be categorized as wet and the latter as dry transaction. But regardless whether its dry or wet , their transaction code must be unique and must never have a chance to have the same transaction code (primary key). If one customer buy a fish its transaction code must be : TRANS001 then if another customer buys a fruts the transaction code must be : TRANS002.