I am trying to create a Reddit type app where the order of a list depends on a combination of the number of upvotes a link has and the created date. My plan is to create a new column in my "Links" table that combines "created_date" and "upvotes" into a "Rank Value" and then sort the list by the "Rank Value".

Is this the right approach? If so, how do I create this table column using ActiveRecord?

If there is a meta attribute that is used purely for display purposes, creating a method that will generate it on the fly would be appropriate.

If you want to use it for sorting your objects as well, it's better to store it in a column. Hopefully, it doesn't depend on things like the current time, and only on its other attributes:

before_save :calculate_rank

def calculate_rank
  self.rank = self.upvotes + self.clicks * 5;
end

Unfortunately, for your use case you specifically said your column depends on the creation date, probably in terms of "how fresh is it" -- a moving target.

You can solve this two ways: by constantly increasing the rank values for newer links indefinitely, or by putting items into time buckets and updating them periodically (degrading their scores when the day or week ends, perhaps).