Select latest model with unique column combination

[fpolli]

I have a database query issue that seems to me like it should be simple because it is a very normal need, but I cannot figure out a way to do it.

I have a table of transactions with a user, a seller, a category and other attributes.

What I need is the most recently updated transaction in each category for a seller-buyer combination.

Please help me. You are my only hope.

Thank you.