variable class attribute access?

I have an array of similar objects which I need to sort based on one their many attributes.
For instance, Campaign has clicks, impressions and cost. I want to be able to dynamically sort by any of those attributes.

def sort_campaign_by(attr)
@campaigns.sort_by{ |c| c.#{attr} }

this (obviously) doesn’t work but hopefully my intentions are clear. I’m trying not to use a case statement - seems unnecessarily lengthy and brittle.

note, I can’t sort them in the AR call, it needs to happen afterwards.

Anyone know how to do this?