Advanced search feature in ruby on rails?

I have a "score" attribute in my "sheet" model which updates daily . and I need to calculate the "average_score(n)" . and add a search feature where the "user" can search for sheets where "average_score" for the last "n" number of days is "greater than" or "less than" or "equal to" 50 or some other number. how can I implement this feature?

Example - find the Sheets where average score for last 3 days [average_score(3)] is > 10