[Help] Set the Value of a Column in DB

I have a few places in my application where doing a basic math equation and setting the value of column would be very useful.

I fudged it by doing it on the view, but then I ran into issues not being able to sort by that value.

I want to do:

column_3 = column_1 + column_2

I'm new enough to Rails to have no idea if this is possible and no amount of Googling has helped me figure it out.

So is it possible?