How to override a column in Rails model?

I have a model for one of my database tables. I want to override the column name for that particular table. How would I achieve it.

For example, let my table be called DUMMY and it has one column called col_a

col_a
20
34
42
23
12

I would be doing a @dummy.col_a. Now this method should return me 0 for numbers ending with 0 and for everything else, it should return the original value. I could do that by defining a new method, but I want to override the column name itself. Please help.