Rails_admin model feature that is a function of other features

I am currently using the rails_admin gem for a project,to create back office for a database and I want to update one of my models. The issue is the new attribute is a function of one of the other attributes and isn't an entry in the database. IE the model has a feature my_id and I want to add a feature dif_id which is a function of my_id.

Any ideas how to do this?